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8.1: Introduction to Angiosperms - Biology

8.1: Introduction to Angiosperms - Biology



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Learning Objectives

  • Compare and contrast monocots and eudicots.
  • Differentiate between monocot and eudicot flowers and leaves.

Of over 400 families of angiosperms, some 80 of them fall into a single clade, called monocots because their seeds have only a single cotyledon. The remainder have seeds that produce two two cotyledons (Figure (PageIndex{1})). This group includes some early diverging angiosperms (ANA grade families and magnoliids), but the large majority of these occupy a single clade called the eudicots. In addition to developmental features, there are a few morphological and anatomical traits you can use to distinguish between these groups.

Monocots

Monocots have a single cotyledon in their seed, parallel venation in their leaves (Figure (PageIndex{2})), flower parts in multiples of three (3-merous, see Figure (PageIndex{3})), and vascular bundles dispersed throughout the stem in concentric circles. Monocots do not have true secondary growth, though some (such as bamboo) form tough, woody stems.

Monocots include:

  • palms (Arecaceae)
  • orchids (Orchidaceae)
  • yams, sweet potatoes (Dioscoreaceae)
  • lilies, onion, asparagus (Liliaceae)
  • bananas (Musaceae)
  • and all the grasses (Poaceae), which include many of our most important plants such as
    • corn (maize)
    • wheat
    • rice
    • and all the other cereal grains upon which we depend so heavily for food as well as
    • sugar cane and bamboo

Eudicots

Eudicots have two cotyledons in their seeds, netted venation in their leaves (Figure (PageIndex{4})), flower parts in multiples of 4 or 5 (4-merous or 5-merous, see Figure (PageIndex{5})), and vascular bundles in the stem arranged in a radial pattern like spokes of a wheel.


  1. Essay on the Introduction to Flowering Plants
  2. Essay on the Angiosperm Derived Characteristics of Flowering Plants
  3. Essay on the Flowering Plant Diversity
  4. Essay on the Fruit and Seed of Flowering Plants

1. Essay on the Introduction to Flowering Plants:

The flowering plants or angiosperms (Angiospermae or Magnoliophyta) are the most diverse group of land plants. The flowering plants and the gymnosperms are the only extant groups of seed plants. The flowering plants are distinguished from other seed plants by a series of apomorphies, or derived characteristics.

The ancestors of flowering plants diverged from gymnosperms around 245-202 million years ago, and the first flowering plants known to exist are from 140 million years ago. They became widespread around 100 million years ago, but replaced conifers as the dominant trees only around 60-100 million years ago.

2. Essay on the Angiosperm Derived Characteristics of Flowering Plants:

The flowers, which are the reproductive organs of flowering plants, are the most remarkable feature distinguishing them from other seed plants. Flowers aid angiosperms by enabling a wider range of adaptability and broadening the ecological niches open to them. This has allowed flowering plants to largely dominate terrestrial ecosystems.

Stamens with Two Pairs of Pollen Sacs:

Stamens are much lighter than the corresponding organs of gymnosperms and have contributed to the diversification of angiosperms through time with adaptations to specialized pollination syndromes, such as particular pollinators. Stamens have also become modified through time to prevent self-fertilization, which has permitted further diversification, allowing angiosperms eventually to fill more niches.

Reduced Male Parts, Three Cells:

The male gametophyte in angiosperms is significantly reduced in size compared to those of gymnosperm seed plants. The smaller pollen decreases the time from pollination — the pollen grain reaching the female plant — to fertilization of the ovary in gymnosperms fertilization can occur up to a year after pollination, while in angiosperms the fertilization begins very soon after pollination. The shorter time leads to angiosperm plants setting seeds sooner and faster than gymnosperms, which is a distinct evolutionary advantage.

Closed carpel enclosing the ovules (carpel or carpels and accessory parts may become the fruit).

The closed carpel of angiosperms also allows adaptations to specialized pollination syndromes and controls. This helps to prevent self-fertilization, thereby maintaining increased diversity. Once the ovary is fertilized, the carpel and some surrounding tissues develop into a fruit. This fruit often serves as an attractant to seed-dispersing animals. The resulting cooperative relationship presents another advantage to angiosperms in the process of dispersal.

Reduced Female Gametophyte, Seven Cells with Eight Nuclei:

The reduced female gametophyte, like the reduced male gametophyte, may be an adaptation allowing for more rapid seed set, eventually leading to such flowering plant adaptations as annual herbaceous life cycles, allowing the flowering plants to fill even more niches.

Endosperm formation generally begins after fertilization and before the first division of the zygote. Endosperm is a highly nutritive tissue that can provide food for the developing embryo, the cotyledons, and sometimes for the seedling when it first appears.

These distinguishing characteristics taken together have made the angiosperms the most diverse and numerous land plants and the most commercially important group to humans. The major exception to the dominance of terrestrial ecosystems by flowering plants is the coniferous forest.

Land plants have existed for about 425 million years. Early land plants reproduced sexually with flagellated, swimming sperm, like the green algae from which they evolved. An adaptation to terrestrialization was the development of upright meiosporangia for dispersal by spores to new habitats. This feature is lacking in the descendants of their nearest algal relatives, the Charophycean green algae.

A later terrestrial adaptation took place with retention of the delicate, a vascular sexual stage, the gametophyte, within the tissues of the vascular sporophyte. This occurred by spore germination within sporangia rather than spore release, as in non-seed plants. A current example of how this might have happened can be seen in the precocious spore germination in Sellaginella, the spike-moss.

The result for the ancestors of angiosperms was enclosing them in a case, the seed. The first seed bearing plants, like the ginkgo, and conifers (such as pines and firs), did not produce flowers. Interestingly, the pollen grains (males) of Ginkgo and cycads produce a pair of flagellated, mobile sperm cells that ‘swim’ down the developing pollen tube to the female and her eggs.

The apparently sudden appearance of relatively modern flowers in the fossil record posed such a problem for the theory of evolution that it was called an ‘abominable mystery’ by Charles Darwin. However, the fossil record has grown since the time of Darwin, and recently discovered angiosperm fossils such as Archaefructus, along with further discoveries of fossil gymnosperms, suggest how angiosperm characteristics may have been acquired in a series of steps.

Several groups of extinct gymnosperms, particularly seed ferns, have been proposed as the ancestors of flowering plants but there is no continuous fossil evidence showing exactly how flowers evolved. Some older fossils, such as the upper Triassic Sanmiguelia, have been suggested. Based on current evidence, some propose that the ancestors of the angiosperms diverged from an unknown group of gymnosperms during the late Triassic (245-202 million years ago).

A close relationship between angiosperms and gnetophytes, proposed on the basis of morphological evidence, has more recently been disputed on the basis of molecular evidence that suggest gnetophytes are instead more closely related to other gymnosperms.

The earliest known angiosperm macrofossil, Archaefructus liaoningensis, is dated to about 125 million years BP (the Cretaceous period), while pollen considered to be of angiosperm origin takes the fossil record back to about 130 million years BP. There is, however, circumstantial chemical evidence for the existence of angiosperms as early as 250 million years ago.

Oleanane, a secondary metabolite produced by many flowering plants, has been found in Permian deposits of that age together with fossils of gigantopterids Gigantopterids are a group of extinct seed plants that share many morphological traits with flowering plants, although they are not known to have been flowering plants themselves.

Recent DNA analysis (molecular systematics) show that Amborella trichopoda, found on the Pacific island of New Caledonia, belongs to a sister group of the other flowering plants, and morphological studies suggest that it has features that may have been characteristic of the earliest flowering plants.

The great angiosperm radiation, when a great diversity of angiosperms appears in the fossil record, occurred in the mid-Cretaceous (approximately 100 million years ago). However, a study in 2007 estimated that the division of the five most recent (the genus Ceratophyllum, the family Chloranthaceae, the eudicots, the magnoliids, and the monocots) of the eight main groups occurred around 140 million years ago.

By the late Cretaceous, angiosperms appear to have dominated environments formerly occupied by ferns and cycadophytes, but large canopy-forming trees replaced conifers as the dominant trees only close to the end of the Cretaceous 65 million years ago or even later, at the beginning of the Tertiary.

The radiation of herbaceous angiosperm occurred much later. Yet, many fossil plants recognizable as belonging to modern families (including beech, oak, maple, and magnolia) appeared already at late Cretaceous.

It is generally assumed that the function of flowers, from the start, was to involve mobile animals in their reproduction processes. That is, pollen can be scattered even if the flower is not brightly colored or oddly shaped in a way that attracts animals however, by expending the energy required to create such traits, angiosperms can enlist the aid of animals and thus reproduce more efficiently.

Island genetics provides one proposed explanation for the sudden, fully developed appearance of flowering plants. Island genetics is believed to be a common source of speciation in general, especially when it comes to radical adaptations that seem to have required inferior transitional forms.

Flowering plants may have evolved in an isolated setting like an island or island chain, where the plants bearing them were able to develop a highly specialized relationship with some specific animal (a wasp, for example).

Such a relationship, with a hypothetical wasp carrying pollen from one plant to another much the way fig wasps do today, could result in both the plant(s) and their partners developing a high degree of specialization. Note that the wasp example is not incidental bees, which apparently evolved specifically due to mutualistic plant relationships, are descended from wasps.

Animals are also involved in the distribution of seeds. Fruit, which is formed by the enlargement of flower parts, is frequently a seed-dispersal tool that attracts animals to eat or otherwise disturb it, incidentally scattering the seeds it contains. While many such mutualistic relationships remain too fragile to survive competition and spread widely, flowering proved to be an unusually effective means of reproduction, spreading (whatever its origin) to become the dominant form of land plant life.

Flower ontogeny uses a combination of genes normally responsible for forming new shoots The most primitive flowers are thought to have had a variable number of flower parts, often separate from (but in contact with) each other.

The flowers would have tended to grow in a spiral pattern, to be bisexual (in plants, this means both male and female parts on the same flower), and to be dominated by the ovary (female part). As flowers grew more advanced, some variations developed parts fused together, with a much more specific number and design, and with either specific sexes per flower or plant, or at least ‘ovary inferior’.

Flower evolution continues to the present day modern flowers have been so profoundly influenced by humans that some of them cannot be pollinated in nature. Many modern, domesticated flowers used to be simple weeds, which only sprouted when the ground was disturbed.

Some of them tended to grow with human crops, perhaps already having symbiotic companion plant relationships with them, and the prettiest did not get plucked because of their beauty, developing a dependence upon and special adaptation to human affection.

The botanical term ‘Angiosperm’, from the Ancient Greek, angeion (receptacle, vessel) and, (seed), was coined in the form Angiospermae by Paul Hermann in 1690, as the name of that one of his primary divisions of the plant kingdom.

This included flowering plants possessing seeds enclosed in capsules, distinguished from his Gymnospermae, or flowering plants with achenial or schizo-carpic fruits, the whole fruit or each of its pieces being here regarded as a seed and naked. The term and its antonym were maintained by Carolus Linnaeus with the same sense, but with restricted application, in the names of the orders of his class Didynamia.

Its use with any approach to its modern scope only became possible after 1827, when Robert Brown established the existence of truly naked ovules in the Cycadeae and Coniferae, and applied to them the name Gymnosperms. From that time onwards, so long as these Gymnosperms were, as was usual, reckoned as dicotyledonous flowering plants, the term Angiosperm was used antithetically by botanical writers, with varying scope, as a group-name for other dicotyledonous plants.

In 1851, Hofmeister discovered the changes occurring in the embryo-sac of flowering plants, and determined the correct relationships of these to the Cryptogamia. This fixed the position of Gymnosperms as a class distinct from Dicotyledons, and the term Angiosperm then gradually came to be accepted as the suitable designation for the whole of the flowering plants other than Gymnosperms, including the classes of Dicotyledons and Monocotyledons. This is the sense in which the term is used today.

In most taxonomies, the flowering plants are treated as a coherent group. The most popular descriptive name has been Angiospermae (Angiosperms), with Anthophyta (‘flowering plants’) a second choice. These names are not linked to any rank. The Wettstein system and the Engler system use the name Angiospermae, at the assigned rank of subdivision.

The Reveal system treated flowering plants as subdivision Magnoliophytina, but later split it to Magnoliopsida, Liliopsida and Rosopsida. The Takhtajan system and Cronquist system treat this group at the rank of division, leading to the name Magnoliophyta (from the family name Magnoliaceae).

The Dahlgren system and Thorne system (1992) treat this group at the rank of class, leading to the name Magnoliopsida. However, the APG system, of 1998, and the APG II system, of 2003, do not treat it as a formal taxon but rather treat it as a clade without a formal botanical name and use the name angiosperms for this clade.

The internal classification of this group has undergone considerable revision. The Cronquist system, proposed by Arthur Cronquist in 1968 and published in its full form in 1981, is still widely used, but is no longer believed to accurately reflect phylogeny.

A general consensus about how the flowering plants should be arranged has recently begun to emerge, through the work of the Angiosperm Phylogeny Group, who published an influential reclassification of the angiosperms in 1998. An update incorporating more recent research was published as APG II in 2003.

Recent studies, as by the APG, show that the monocots form holophyletic or monophyletic group this clade is given the name monocots. However, the dicots are not (they are a paraphyletic group). Nevertheless, within the dicots a monophyletic group does exist, called the eudicots or tricolpates, and including most of the dicots. The name tricolpates derives from a type of pollen found widely within this group.

The name eudicots is formed combining dicot with the prefix eu- (from Greek, for ‘well’, or ‘good’, botanically indicating ‘true’), as the eudicots share the characters traditionally attributed to the dicots, such as flowers with four or five parts (four or five petals, four or five sepals).

Separating this group of eudicots from the rest of the (former) dicots leaves a remainder, which sometimes are called informally palaeodicots (Greek prefix ‘palaeo-‘means ‘old’). As this remnant group is not monophyletic this is a term of convenience only.

3. Essay on the Flowering Plant Diversity:

The number of species of flowering plants is estimated to be in the range of 250,000 to 400,000. The number of families in APG (1998) was 462. In APG II (2003) it is not settled at maximum it is 457, but within this number there are 55 optional segregates, so that the minimum number of families in this system is 402.

The diversity of flowering plants is not evenly distributed. Nearly all species belong to the eudicot (75%), monocot (23%) and magnoliid (2%) clades. The remaining 5 clades contain a little over 250 species in total, i.e., less than 0.1 per cent of flowering plant diversity, divided among 9 families.

The amount and complexity of tissue-formation in flowering plants exceeds that of Gymnosperms. The vascular bundles of the stem are arranged such that the xylem and phloem form concentric rings.

In the Dicotyledons, the bundles in the very young stem are arranged in an open ring, separating a central pith from an outer cortex. In each bundle, separating the xylem and phloem, is a layer of meristem or active formative tissue known as cambium. By the formation of a layer of cambium between the bundles (inter-fascicular cambium) a complete ring is formed, and a regular periodical increase in thickness results from the development of xylem on the inside and phloem on the outside.

The soft phloem becomes crushed, but the hard wood persists and forms the bulk of the stem and branches of the woody perennial. Owing to differences in the character of the elements produced at the beginning and end of the season, the wood is marked out in transverse section into concentric rings, one for each season of growth, called annual rings.

Among the Monocotyledons, the bundles are more numerous in the young stem and are scattered through the ground tissue. They contain no cambium and once formed the stem increases in diameter only in exceptional cases.

Traditionally, the flowering plants are divided into two groups, which in the Cronquist system are called Magnoliopsida (at the rank of class, formed from the family name Magnoliacae) and Liliopsida (at the rank of class, formed from the family name Liliaceae).

Other descriptive names allowed by Article 16 of the ICBN include Dicotyledones or Dicotyledoneae, and Monocotyledones or Monocotyledoneae, which have a long history of use. In English a member of either group may be called a dicotyledon (plural dicotyledons) and monocotyledon (plural monocotyledons), or abbreviated, as dicot (plural dicots) and monocot (plural monocots).

These names derive from the observation that the dicots most often have two cotyledons, or embryonic leaves, within each seed. The monocots usually have only one, but the rule is not absolute either way. From a diagnostic point of view the number of cotyledons is neither a particularly handy nor reliable character.

The characteristic feature of angiosperms is the flower. Flowers show remarkable variation in form and elaboration, and provide the most trustworthy external characteristics for establishing relationships among angiosperm species. The function of the flower is to ensure fertilization of the ovule and development of fruit containing seeds. The floral apparatus may arise terminally on a shoot or from the axil of a leaf (where the petiole attaches to the stem).

Occasionally, as in violets, a flower arises singly in the axil of an ordinary foliage-leaf. More typically, the flower-bearing portion of the plant is sharply distinguished from the foliage- bearing or vegetative portion, and forms a more or less elaborate branch-system called an inflorescence.

The reproductive cells produced by flowers are of two kinds. Microspores, which will divide to become pollen grains, are the ‘male’ cells and are borne in the stamens (or microsporophylls). The ‘female’ cells called megaspores, which will divide to become the egg-cell (megagameto- genesis), are contained in the ovule and enclosed in the carpel (or megasporophyll).

The flower may consist only of these parts, as in willow, where each flower comprises only a few stamens or two carpels. Usually other structures are present and serve to protect the sporophylls and to form an envelope attractive to pollinators. The individual members of these surrounding structures are known as sepals and petals (or tepals in flowers such as Magnolia where sepals and petals are not distinguishable from each other).

The outer series (calyx of sepals) is usually green and leaf-like, and functions to protect the rest of the flower, especially the bud. The inner series (corolla of petals) is generally white or brightly coloured, and is more delicate in structure. It functions to attract insect or bird pollinators.

Attraction is effected by colour, scent, and nectar, which may be secreted in some part of the flower. The characteristics that attract pollinators account for the popularity of flowers and flowering plants among humans.

While the majority of flowers are perfect or hermaphrodite (having both male and female parts in the same flower structure), flowering plants have developed numerous morphological and physiological mechanisms to reduce or prevent self-fertilization.

Heteromorphic flowers have short carpels and long stamens, or vice versa, so animal pollinators cannot easily transfer pollen to the pistil (receptive part of the carpel). Homomorphic flowers may employ a biochemical (physiological) mechanism called self- incompatibility to discriminate between self-and non-self-pollen grains. In other species, the male and female parts are morphologically separated, developing on different flowers.

Double fertilization refers to a process in which two sperm cells fertilize cells in the ovary. This process begins when a pollen grain adheres to the stigma of the pistil (female reproductive structure), germinates, and grows a long pollen tube. While this pollen tube is growing, a haploid generative cell travels down the tube behind the tube nucleus.

The generative cell divides by mitosis to produce two haploid (n) sperm cells. As the pollen tube grows, it makes its way from the stigma, down the style and into the ovary. Here the pollen tube reaches the micropyle of the ovule and digests its way into one of the synergids, releasing its contents (which include the sperm cells).

The synergid that the cells were released into degenerates and one sperm makes its way to fertilize the egg cell, producing a diploid (2n) zygote. The second sperm cell fuses with both central cell nuclei, producing a triploid (3n) cell. As the zygote develops into an embryo, the triploid cell develops into the endosperm, which serves as the embryo’s food supply. The ovary now will develop into fruit and the ovule will develop into seed.

4. Essay on the Fruit and Seed of Flowering Plants:

As the development of embryo and endosperm proceeds within the embryo-sac, the sac wall enlarges and combines with the nucellus (which is likewise enlarging) and the integument to form the seed-coat. The ovary wall develops to form the fruit or pericarp, whose form is closely associated with the manner of distribution of the seed.

The character of the seed-coat bears a definite relation to that of the fruit. They protect the embryo and aid in dissemination they may also directly promote germination. Among plants with indehiscent fruits, the fruit generally provides protection for the embryo and secures dissemination. In this case, the seed-coat is only slightly developed. If the fruit is dehiscent and the seed is exposed, the seed-coat is generally well developed, and must discharge the functions otherwise executed by the fruit.

Agriculture is almost entirely dependent on angiosperms, either directly or indirectly through livestock feed. Of all the families plants, the Poaceae, or grass family, is by far the most important, providing the bulk of all feed stocks (rice, corn – maize, wheat, barley, rye, oats, pearl millet, sugar cane, sorghum). The Fabaceae, or legume family, comes in second place.

Also of high importance are the Solanaceae, or nightshade family (potatoes, tomatoes, and peppers, among others), the Cucurbitaceae, or gourd family (also including pumpkins and melons), the Brassicaceae, or mustard plant family (including rapeseed and cabbage), and the Apiaceae, or parsley family. Many of our fruits come from the Rutaceae, or rue family, and the Rosaceae, or rose family (including apples, pears, cherries, apricots, plums, etc.).

In some parts of the world, certain single species assume paramount importance because of their variety of uses, for example the coconut (Cocos nucifera) on Pacific atolls, and the olive (Olea europaea) in the Mediterranean region.

Flowering plants also provide economic resources in the form of wood, paper, fiber (cotton, flax, and hemp, among others), medicines (digitalis, camphor), decorative and landscaping plants, and many other uses. The main area in which they are surpassed by other plants is timber production.


Flowers

Flowers are modified leaves, or sporophylls, organized around a central receptacle. Although they vary greatly in appearance, virtually all flowers contain the same structures: sepals, petals, carpels, and stamens. The peduncle typically attaches the flower to the plant proper. A whorl of sepals (collectively called the calyx ) is located at the base of the peduncle and encloses the unopened floral bud. Sepals are usually photosynthetic organs, although there are some exceptions. For example, the corolla in lilies and tulips consists of three sepals and three petals that look virtually identical. Petals , collectively the corolla , are located inside the whorl of sepals and may display vivid colors to attract pollinators. Sepals and petals together form the perianth . The sexual organs, the female gynoecium and male androecium are located at the center of the flower. Typically, the sepals, petals, and stamens are attached to the receptacle at the base of the gynoecium, but the gynoecium may also be located deeper in the receptacle, with the other floral structures attached above it.

As illustrated in (Figure), the innermost part of a perfect flower is the gynoecium , the location in the flower where the eggs will form. The female reproductive unit consists of one or more carpels, each of which has a stigma, style, and ovary. The stigma is the location where the pollen is deposited either by wind or a pollinating arthropod. The sticky surface of the stigma traps pollen grains, and the style is a connecting structure through which the pollen tube will grow to reach the ovary. The ovary houses one or more ovules, each of which will ultimately develop into a seed . Flower structure is very diverse, and carpels may be singular, multiple, or fused. (Multiple fused carpels comprise a pistil .) The androecium , or male reproductive region is composed of multiple stamens surrounding the central carpel. Stamens are composed of a thin stalk called a filament and a sac-like structure called the anther . The filament supports the anther, where the microspores are produced by meiosis and develop into haploid pollen grains, or male gametophytes.



Modes of Reproduction in Angiosperms (With Diagrams) | Botany

Let us make an in-depth study of Modes of Reproduction in Angiosperms. After reading this article you will learn about: 1. Introduction to Modes of Reproduction 2. Various Modes of Reproduction in Angiosperms.

Introduction to Modes of Reproduction:

In angiosperms or flowering plants, there are several modes of reproduction. Generally, they are arranged in two large groups of reproduction, i.e., (i) asexual or vegetative and (ii) sexual types.

In asexual or vegetative reproduction, the offspring are produced from the somatic cells, while in sexual reproduction there is fusion of male and female gametes.

In the case of vegetative reproduction, any part of the plant, i.e., stem, root or leaf, is capable of growing into a new plant, in addition to performing its sexual functions. Sometimes, in certain plants, buds and bulbils are developed, which develop into new plants.

In sexual reproduction, the gametes from male and female organs of the flower are fused to produce a zygote. In angiosperms, these organs are generally called, androecium and gynoecium, respectively.

In some plants, certain special modes of reproduction are found, which are commonly known as parthenogenesis, sporophytic budding, polyembryony, apomixis, apospory, and micro- propagation. The production of synthetic or artificial seeds is also possible through tissue culture.

Various Modes of Reproduction in Angiosperms:

Sexual Reproduction:

The flower is a highly specialized reproductive shoot. Each typical flower consists of four distinct types of members arranged in four separate but closely set whorls, one above the other, on the top of a long or short stalk. The lower two whorls are called accessory whorls, and the upper two essential or reproductive whorls because only these two are directly concerned in reproduction.

The essential whorls consist of two kinds of sporophylls: microsporophylls or stamens and megasporophylls or carpels. Both kinds of sporophylls may be present in a flower (hermaphrodite flower), or only one (unisexual flower) may be seen in some types.

This is male whorl of the flower. It consists of stamens or microsporophylls which are regarded as the male organ of the flower. Each stamen consists of three parts, i.e., filament, anther and connective. The anther filament is the slender stalk of the stamen, and the anther is the expanded head borne by the filament at its tip.

Each consists usually of two lobes connected together by a sort of mid-rib, the connective. The anther bears four chambers or pollen sacs each filled with pollen grains or microspores. Pollen grains are produced in large quantities in the pollen sacs.

This is female whorl, and its component parts are known as carpels or megasporophylls. The gynoecium or pistil consists of ovary, style and stigma. The swollen basal part of the gynoecium which forms one or more chambers is known as ovary. The small rounded head of the gynoecium is known as the stigma.

The ovary contains one or more little, roundish or oval, egg-like bodies which are the rudiments of seeds and are known as ovules. Each ovule encloses a large oval cell known as the embryo sac. On maturation, the ovary gives rise to the fruit and the ovules to seeds.

Embryo sac = Female gametophyte

Anther (pollen sac) = Microsporangium

Germinating pollen grain = Male gametophyte

The reproductive process in angiosperms is as follows:

Development of the Male Gametophyte:

Microsporangium (pollen sac):

The epidermis is the outermost layer of microsporangium. The cells of epidermis are generally stretched and flattened. The layer next to the epidermis is the endothecium or fibrous layer. As a rule, by the development of the fibrous bands of thickening the endothecium becomes hygroscopic and is, therefore, mainly responsible for the dehiscence of mature anther.

The anther is generally bilobed, containing two longitudinally running chambers or pollen sacs per lobe. Each chamber contains a large number of pollen grains. The anther wall is composed of 4 to 5 layers. The innermost layer of these wall layers develops into a single layered tapetum.

The tapetal layer is of great physiological significance as all the food material entering into the sporogenous tissue diffuses through, this layer. Ultimately the cells of tapetal layer disorganise. Thus, tapetum makes a nutritive layer for the developing microspores.

The cells of endothecium are thin walled along the line of dehiscence of each anther lobe. The opening through which the pollen grains are discharged from the pollen sac is called stomium.

On the maturity of the anther, a strain is exerted on the stomium due to the loss of water by the cells of endothecium, with the result the stomium ruptures and the anther dehisces. Generally, the mature anther dehisces by means of slits or apical pores.

The pollen grains or microspores are the male reproductive bodies of a flower, and are contained in the pollen sac or microsporangia. They are very minute in size and are like particles of dust. Each pollen grain consists of a single microscopic cell, possessing two coats: the exine and the intine.

The exine, tough cutinized layer, which is often provided with spinous outgrowths or reticulations of different patterns and sometimes smooth. The exine is made up of a complex substance, called sporopollenin. The inline is a thin, delicate cellulose layer lying internal to the exine. The exine possesses one or more thin places known as germ pores. There are usually three germ pores in dicots and one in monocots.

The development of the male gametophyte is remarkably uniform in angiosperms. Pollen grain is the first cell of a male gametophyte. This cell undergoes only two divisions, with the result of first division two cells are formed: a large vegetative cell and a small generative cell.

The second division is concerned with generative cell only. This division may take place either in the pollen grain or in the pollen tube, and give rise to two male gametes. The life of male gametophyte is very short as compared to that of the sporophyte.

Vegetative and Generative Cells:

As already described, the first division of the pollen grain gives rise to the vegetative and generative cells. The first formed walled and peripheral cell is the generative cell, while the larger, naked, central cell, which fills the remainder of the spore-wall cavity, is the vegetative or tube cell.

The nuclei of generative and vegetative cells differ in size, structure and in staining qualities. The nucleus of vegetative cell possesses a prominent nucleus, while the nucleus of generative cell contains a small nucleolus. The cytoplasm of generative cell is hyaline and is almost without RNA, whereas that of vegetative cell is rich in RNA.

The DNA contents of both the nuclei are same in the beginning but later on they increase in the generative nucleus. Eventually the generative cell loses contact with the microspore (pollen grain) wall, and is being shifted into the vegetative cell, where it may lie in any part of it.

Thus, the pollen grain becomes two-celled. Generally, the pollen grains are being shed from the microsporangium (pollen sac) in two-celled stage for pollination.

Development of the Female Gametophyte:

The megasporangium or ovule:

An ovule or megasporangium develops from the base or the inner surface of the ovary. It is a small generally oval structure and consists chiefly of a central body of tissue, the nucellus and one or two integuments. Each ovule is attached in the placenta by a small stalk called the funiculus.

The place of attachment of the stalk with the main body of the ovule is called the hilum. In an inverted ovule, the funicle fuses with the main body of the ovule, forming a sort of ridge, known as the raphe. The upper end of the raphe which is the .unction of the integuments and the nucellus is called the chalaza.

The nucellus makes the main body of the ovule, which is made up of parenchyma tissue. Nucellus is the megasporangium proper and is surrounded by two coats, the integuments. A small opening is left at the apex of the integuments this is called the micropyle.

When there are two integuments then the inner integument is formed first and followed by the formation of the outer integument. A large oval cell lying embedded in the nucellus towards the micropylar end is the embryo sac. This makes the most important part of the mature ovule. It is the embryo sac, which bears the embryo later on.

The placenta is an outgrowth of a parenchymatous tissue in the inner wall of the ovary to which the ovule or ovules (megasporangia) remain attached. The placentae usually develop on the margins of carpels, either along their whole line of union, called the suture or at their base or apex. The manner in which the placentae are distributed in the cavity of the ovary is known as placentation.

In the simple ovary (i.e., of one carpel), there is one common type of placentation, known as marginal, and in the compound ovary (i.e., of two or more carpels united together) placentation may be axile, central and free-central, basal, parietal and superficial as shown in fig. 46.14.

P. Maheshwari, F.R.S. (1904-1966):

He worked on embryological aspects, especially the embryo sac of several plants belonging to more than 100 angiospermous families.

His famous books of international fame are:

1. Introduction to the Embryology of Angiosperms

2. Recent advances in Embryology of Angiosperms (1963, edited by P. Maheswari).

He devoted his life for plant embryology, and very often referred to as ‘Father of Indian Plant Embryology.’ He was honoured with fellowship of Royal Society.

The archesporium is hypodermal in origin. At some early stage in the development of the ovule, usually at the time of the initiation of the integumentary primordia, single hypodermal cell known as primary archesporial cell, becomes differentiated at the apex of the nucellus beneath the epidermis.

It can be distinguished from other neighbouring cells owing to its large size, conspicuous deeply staining nucleus, and dense cytoplasm. Usually this primary archesporial cell divides periclinically forming an outer primary parietal cell and an inner primary sporogenous cell.

The primary parietal cell may divide further several times both by anticlinal and periclinical divisions forming a variable amount of parietal tissue, or sometimes it remains undivided. The primary sporogenous cell usually does not divide further and functions directly as the megaspore mother cell.

Usually the megaspore mother cell divides meiotically forming a tetrad of four megaspores. This usual process of meiotic division is termed megasporogenesis. Here the first division (i.e., meiosis I) is always transverse and gives rise to two cells.

The second is also transverse (i.e., meiosis II), and thus in total four cells are being formed. The four megaspores thus formed in an axial row within the nucellus forming a linear tetrad.

Of the four megaspores, so formed, each with half (n) the usual number (2n) of chromosomes, the three upper ones degenerate and appear as dark caps, while the lowest one functions, and gives rise to the embryo sac. The developing megaspore encroaches upon and absorbs the other three degenerating megaspores of tetrad and the neighbouring cells of the nucellus.

The megaspore (n) makes the beginning of the female game­tophyte. The nucleus of the functional megaspore divides and develops into the female gametophyte or the embryo sac. The female gametophyte of angiosperms is very much reduced and totally dependent for its nutrition upon the tissue of the sporophyte.

Depending on the number of megaspores taking part in the development, the embryo sacs (female gametophytes) of angiosperms may be classified into three main categories monosporic, bisporic and tetrasporic (Panchanan Maheshwari, 1950).

In monosporic type, only one of the four megaspores takes part in the development of the female gametophyte (embryo sac). In bisporic type, two megaspore nuclei take part in the develo­pment of the female gametophyte. However, in tetrasporic type, all the four megaspore nuclei take part in the development of female gametophyte.

The functional megaspore (n) is the first cell of the female gametophyte. It divides by three successive divisions to form an eight-nucleate female gametophyte or embryo sac. Here, the nucleus of the functional megaspore divides to form two nuclei the primary micropylar and the primary chalazal nuclei.

These nuclei again divide, so that the number is increased to four. Each of these nuclei divides, so that altogether eight nuclei are formed in the embryo sac, four at each end. The female gametophyte increases in size. Now, one nucleus from each end or pole passes inwards, and the two polar nuclei fuse together somewhere in the middle of the embryo-sac, forming the secondary nucleus (2n).

The remaining three nuclei at the micropylar end, each surrounded by a very thin wall, form the egg apparatus. The egg apparatus consists of two synergids and an egg cell. The other three nuclei at the opposite or chalazal end, lying in a group, often surrounded by very thin walls, form the antipodal cells. This type of embryo-sac is the most common and generally known as the normal type.

Pollination:

In angiosperms, the pollen grains are being transferred from the anther to the stigma, and is termed pollination. This phenomenon was first discovered by Camerarius (1694) in the end of seventeenth century. According to him, pollination is essential for the production of the seed.

The pollination may be of two types: self pollination (autogamy) and cross pollination (allogamy). The transfer of the pollen-from the anther of a flower to the stigma of the same flower is known as self pollination or autogamy, whereas cross pollination or allogamy is the transference of the pollen from one flower to another flower.

The cross pollination is of three types:

In this type, the pollination takes place between flowers borne on two different plants of the same species.

This type of polli­nation takes place between the flowers developed on the same plant.

Such pollination takes place between two flowers of two different plants of the allied species or sometimes even allied genera.

In the condition in which the pollen are discharged from the anther, they show consi­derable resistance to environmental changes. Sometimes, they remain viable for several weeks.

In certain cases even in hermaphrodite flowers, self-pollination does not take place. This happens because of heterostyly, e.g., in Primula vulgaris dichogamy, where the maturity of male and female sex organs of the flowers is attained at different times, e.g., in Impatiens- herkogamy, in which the structure of male and female sex organs in the flowers acts as barrier for self-pollination, and self-sterility, as found in Petunia axillaris.

As mentioned, pollination is of two kinds:

(1) Self pollination or autogamy (auto = self gamos = marriage) and

(2) Cross-pollination or allogamy (alios = different).

1. Self-Pollination (Autogamy):

This kind of pollination is the transference of pollen grains from the anther of a flower to the stigma of the same flower or from a flower (male or bisexual) to a flower (female or bisexual), both found on the same individual plant.

Here only one parent plant is concerned to give rise to the offspring. The self-pollination is, however, presented in unisexual flowers borne by two separate plants, and also in many bisexual flowers. The under mentioned adaptations are commonly found in flowers to achieve self-pollination.

In this condition, the anthers and the stigmas of a bisexual flower mature at the same time:

(i) Here some of the pollen grains may reach the stigma of the same flower through the agency of wind or insects, thus effecting self-pollination.

(ii) The filaments of the anthers may recoil and bring the mature anther close to the stigma (e.g., in Mirabilis jalapa). The anthers then burst and discharge their pollen right on the surface of the stigma. In some cases, the stigmas move back and touch the anthers to achieve self-pollination when cross-pollination fails (e.g., in members of Asteraceae and Malvaceae families).

(iii) In some drooping flowers the style is longer than the filaments, whereas in certain erect flowers the reverse may be the case.

(iv) Sessile or sub-sessile anthers may lie at the mouth of the narrow corolla tube and the stigma, while pushing out through the tube brushes against anthers (e.g.. in Ixora, Gardenia and Vinca).

Cleistogamy (kleistos = closed):

Some of the bisexual flowers do not open and are known as cleistogamous or closed flowers. In such flowers, the pollen grains are distributed on the stigma of the same flowers. Such cleistogamous flowers are very small and inconspicuous.

They are not coloured, and do not secrete any nectar. These flowers are not even scented. This type of pollination is found in the underground flowers of Commelina benghalensis, Viola, Drosera, Oxalis, etc.

2. Cross-Pollination (Allogamy):

The cross-pollination is induced by external agents which carry the pollen grains of one flower and deposit them on the stigma of another flower, the two being borne by two separate plants of the same or closely allied species. These agencies may be insects (e.g., bees, flies, moths, etc.), animals (e.g., birds, snails, etc.), wind and water.

The allogamy (cross-pollination) is the rule in unisexual flowers borne by two separate plants, while in bisexual flowers, it also occurs generally. Nature favours cross-pollination and there are so many adaptations in flowers to achieve this type of pollination.

Entomophily (entomon = insect, phileo = to love):

This type of pollination takes place through the agency of insects. It is of general occurrence among plants. The insect-loving flower possesses various adaptations by which they attract insects and use them as carriers of pollen grains for the purpose of cross-pollination.

The main such adaptations are colour, nectar and scent. The flowers of Asteraceae and Lamiaceae families are generally pollinated by the bees and butterflies.

Generally, the pollen grains of entomophilous flowers are sticky. The stigma is also sticky. Pollen grains and nectar are very often used as food materials by the insects. Flowers generally attract insects by their colour, nectar or scent, or they visit the flowers in search of food, or shelter from sun and rain.

Thus, as the insects visit the flowers, their body gets dusted with pollen grains, and when they fly and visit other flowers, they brush against the stigma which being sticky at once receives the pollen grains from their body. Thus, cross-pollination is achieved.

In several species of Ficus, the insects enter the chamber of the inflorescence (hypanthodium) through the apical pore, and as they move over the unisexual flowers inside the chamber, the pollination is achieved. Female flowers lie at the base of the cavity and open earlier, whereas male flowers lie near the apical opening and open later so that pollen grains have to be brought over from another inflorescence.

Flowers are generally adapted for pollination by some specific insects. For example, in snapdragon and other such flowers with saccate corolla, only the insects of particular size and weight can open the mouth of the corolla. On the other hand, long-tongued insects can pollinate the flowers with long corolla tubes.

Pollination in Calotropis:

This is member of Asclepiadaceae and is pollinated by bees. In this flower, the filaments of stamens form a tube around gynoecium. The anthers are fused with the stigma to form a 5-angled disc called gynostegium.

The staminal tube gives out distinct lobes called the corona. It is fused with the petals. The anther lobes, that are fused with the stigmatic disc, have straight and parallel sides and are separated only by long narrow clefts.

Within these clefts there are inter-staminal chambers. The pollen grains in the anthers are grouped in the form of mass called pollinium. The pollinium develops a rider mechanism or the translator. It consists of two arms called the caudicles. The caudicles carry the two pollinia on one side and are fused to form a black and sticky dot on the other side. This dot-like structure is called the corpusculuni.

There are five corpuscula at the angles of gynostegium from two adjacent anthers. An insect crawling about over the flowers is sooner or later trapped, through one of its legs becoming caught in one of the clefts between adjacent anthers.

The insect can release itself only by drawing the leg upwards through the clefts and this it does, but as the leg becomes free at the top of the cleft, it catches in the notch of the corpusculum so that further movements pull this together with its attached pollinia, away from its anchorage on the gynostegium.

The released insect in due course visits another flower and again becomes caught by the leg in the similar way. While drawing the leg, this time, through the anther cleft the pollinia brought from the previous flower are torn away from corpusculum and are deposited in the inter-staminal chamber. The new translators are carried away and there is repetition of the whole process.

Pollination in Salvia:

An interesting type of cross-pollination takes place in Salvia by insects. In this flower, there are two stamens. The two anther lobes of each stamen are widely separated by the elongated curved connective which plays freely on the filament. The upper lobe is fertile and the lower one sterile. In the natural position, the connective remains upright.

When the insect enters the tube of the corolla it pushes the lower sterile anther lobe of each stamen the connective swings round with the result that the upper fertile lobe comes down and strikes the back of the insect with force and dusts it with pollen grains.

The flower is protandrous, and on the maturity of the stigma it bends down and touches the back of the insect and receives the pollen grains from it. Thus, pollination is effected.

Anemophily (anemos = wind):

In many cases, pollination is achieved by wind. The wind pollinated flowers are small and inconspicuous. They are neither coloured nor showy. They do not have any smell and they do not secrete any nectar. The anthers produce an immense quantity of pollen grains. A large quantity of pollen grains is being wasted during transit from one flower to another.

The pollen grains are quite light and dry, and sometimes provided with wings (e.g., in Pinus, a gymnosperm) to facilitate distribution by wind. In the wind loving flowers, the stigmas are comparatively large and protruding, some-times branched and often feathery (e.g., grasses, bamboos, palms, cereals, millets, sedges, sugarcane, etc.).

The maize plant makes a good example of this type. The plant bears a large number of male flowers in a terminal panicle, and in the lower part of the plant one or two female spadices, each in the axil of a leaf, surrounded by spathes. A cluster of fine, silky and long styles is seen.

On the maturity, the anthers burst and a cloud of dust-like pollen grains is seen floating in the air near the plant. Some of these pollen grains are entangled by the protruding stigmas and thus pollination is effected.

Hydrophily (hydor = water):

Water also acts as an agent of pollination. This is commonly found in water plants, specially submerged ones, such as Vallisneria, Ceratophyllum, Hydrilla and Zostera. The mode of pollination in Vallisneria (submerged aquatic plant) is as follows:

The plant is dioecious. The male plant bears a large number of minute male flowers in a small spadix surrounded by a spathe and borne on a short stalk, whereas the female plant bears solitary female flowers each on a long slender pedicel.

This stalk of the flower elongates and takes the female flower to the surface of the water. The spathe bursts releasing the male flowers from the spadix, while still closed, and float on the surface of the water. The anthers burst and the sticky pollen grains adhere to the surface of trifid stigmas which thereafter close.

As soon the pollination is over, the stalk of the female flower becomes spirally coiled and pulls the female flower down into the water. The fruit develops and matures under water a little above the bottom.

Zoophily (zoo = animals):

There are so many animals, such as birds, squirrels, bats, snails, etc., which are involved in cross-pollination. The pollination by birds, generally called ornithophily, is common in coral tree (Erythrina), bottle brush (Callistemon), Butea monosperma and silk cotton tree (Bombax ceiba). The pollination in Adansonia, Kigelia and Anthocephalus are carried out by bats.

This type is called chiropterophily. Snails are involved in pollination of several aroids (members of family Araceae). In aroids, the inflorescence is a spadix the female flowers remain situated at the base of the spadix and the male flowers towards top. The stigmas mature first and the pollen grains are brought from another spadix.

Significance of Pollination:

1. Pollination is an important process which leads to fertilisation and production of seeds and fruits, which are responsible for continuity of plant life.

2. The seeds and fruits are also used as food both for animal and humans. They make source of vitamins and minerals.

3. The pollination, especially cross pollination is important for production of plants with a combination of characters from two plants.

4. Pollination is also important in the production of hybrid seeds.

Contrivances for cross-pollination:

Certain structural devices in the flowers favour cross-pollination.

The stamens and carpels lie in separate flowers—male and female, either borne by the same plant or by two separate plants.

There are two kinds of unisexuality:

(i) Where the male and female flowers lie on the same plan’ and the plant is said to be monoecious (e.g., members of Cucurbitaceae, castor, maize, etc.),

(ii) Where the male and the female flowers are borne by one plant and the female flowers lie on another plant, it is known as dioecious (e.g., palmyra palm, Carica papaya, Morus alba, etc.). In monoecious plants, there may be self-or cross-pollination, while in dioecious plants, cross-pollination is a basic necessity.

2. Self-Sterility:

In certain flowers, the pollen grains are unable to germinate on its own stigma. It is noted in some orchids that the pollen has an injurious effect on the stigma of the same flower. In this case on the application of pollen to stigma, the stigma dries up and falls off, Abutilon, Passiflom, Malva, Prunus and Pyrus are self-sterile.

To effect the successful cross- pollination in these cases the pollen must be from two such parents which differ genetically. Cross-pollination is the only method in such cases for the setting of seeds.

3. Dichogamy (dicha = in two):

In many bisexual flowers, the anthers and stigmas often mature at different times. This condition is known as dichogamy. As the anther and the stigma mature at different times, dichogamy often checks the self-pollination.

There are two types of dichogamy:

(i) Protogyny (protos = first gyne = female) where the gynoecium matures earlier than the anthers of the same flower, and in such cases, the stigma receives the pollen grains brought from another flower and thus cross- pollination becomes indispensable (e.g., Ficus, Mirabilis, Magnolia, Annona, etc.) and

(ii) Protandry (protos = first andros = male) where the anthers mature earlier than the stigma of the same flower and hence the pollen grains, are carried over to the stigma of another flower (e.g., Clerodendron, Hibiscus rosa-sinensis, Abelmoschus esculentus, Helianthus annuus, Coriandrum sativum, etc.)

4. Heterostyly (heteros = different):

Some plants bear flowers of two different forms. One form possesses long stamens and a short style, while the other form possesses short stamens and a long style. This kind of bearing of stamens and styles is known as dimorphic heterostyly. In such cases, the chances of self-pollination decrease whereas chances of cross-pollination increase.

In the flowers of this type, cross-pollination readily takes place between stamens and styles of the same length borne by different flowers. Dimorphic heterostyly is seen in Oxalis, Linum, Polygonum fagopyrum, Woodfordia, etc.

5. Herkogamy (hercos = barrier):

In some homogamous flowers, there are certain structural peculiarities of the floral parts which act as a barrier to self-pollination and thus favour cross- pollination by insects.

Here are cited some important examples:

For example, in Calotropis and orchids, the pollinia are located at places where they can never come in contact with the stigma by themselves and can only be carried away by insects. The lever mechanism in Salvia also promotes cross-pollination and avoids self-pollination.

In Viola tricolor the stigma is protected by a flap or a lid that prevents contact between the stigma and anther. This flap is pushed aside by the insect and thus cross-pollination is effected.

Some of more important features are given here:

Advantages of self-pollination:

This type of pollination is almost certain in a bisexual flower, if the stamens and carpels of the flower mature at the same time.

Disadvantages of self-pollination:

Continued self-pollination generation after generation definitely results in weaker progeny.

Advantages of cross-pollination:

(i) It always gives rise to healthier offspring in subsequent generations which are better adapted in the struggle for existence

(ii) More abundant and viable seeds are produced

(iii) New varieties can be developed by this method

(iv) The adaptability of the plants to their environment is definitely better by this method.

Disadvantages of cross-pollination:

(i) The plants have to depend upon external agencies for pollination (such as wind, water, insects, and animals)

(ii) Various devices are needed to fulfil the needs of outer agencies

(iii) There is always a considerable waste of pollen where wind is the pollinating agent in cross-pollination.

Pollen Germination and Fertilisation:

The fusion of two dissimilar sexual reproductive units or gametes is termed as fertilization. In gymnosperms, the pollen grains usually land directly on the nucellus, while in angiosperms, they fall on the stigma.

In angiosperms, the fertilization is being completed as follows:

Germination of pollen grain:

After being deposited on the stigma, the pollen grain absorbs liquid from the moist surface of the stigma, expands in size, and the intine protrudes out through a germ pore. The small tubular structure also known as pollen tube continues to elongate, and makes its way down the tissues of the stigma and style. Only the distal part of the pollen tube possesses living cytoplasm.

The stigma plays an important role in the germination of pollen grain. The stigma secretes fluid containing liquids, gums, sugar and resins. The chief function of the stigmatic secretion is to protect the pollen as well as the stigma from desiccation.

After arriving to the wall of the ovary, the pollen tube enters the ovule either through the micropyle or by some other route. The entrance of the pollen tube through the micropyle is the normal condition and is known as porogamy. In some cases the pollen tube enters the ovule through the chalaza. This condition is known as chalagogamy. The chalagogamy was first reported by Treub (1891), in Casuarina.

The period between pollination and fertilisation varies from plant to plant. Usually this period varies from 12 to 48 hours. According to P. Maheshwari (1949), the temperature is responsible for controlling the growth of pollen tube.

In normal case, one male gamete unites with the egg to form the zygote and the second travels a little farther and unites with the secondary nucleus. This process is known as double fertilisation (Navaschin, 1898 Guignard, 1899).

As the second male gamete fuses with the secondary diploid (2n) nucleus, producing a triploid (3n) primary endosperm nucleus, this is called triple fusion. Thus in an embryo sac there occur two sexual fusions one in syngamy (i.e., fusion of male gamete with egg), and the other in triple fusion, and therefore, the phenomenon is called double fertilisation.

Significance of Double Fertilization:

The embryo, during its growth and development receives its nourishment from endosperm, which is a product of double fertilization. This process also gives required energy to the polar nuclei, which fail to divide further.

Since the endosperm nuclei are the resultants of double fertilization, they are characterized by maternal and paternal chromosomes and thus endosperm represents the physiological aggressiveness due to hybrid vigour.

The fusion of male and female gametes as well as double fertilization are equally responsible for the production of the viable seeds because absence of any one of these two may cause lethal effect, and the viable seeds are not produced.

Pollination and Fertilization in Vitro:

In many crosses, sometimes plant breeders face the problems such as the failure of the germination of the pollen, short length of pollen tube, or a slow growth of the pollen tube. The direct injection of pollen grains into the ovary may be helpful to overcome such problems. Besides the technique of direct injection of pollen suspension into the ovary, in vitro pollination of pistils has also been accomplished.

Un-pollinated ovaries of Nicotiana rustica were grown on Nitsch’s medium containing 4 per cent sucrose and pollinated the next day with pollen collected from dehiscing anthers. The process of fertilization and the development of endosperm and embryo were normal and mature seeds were obtained (P.S. Rao, 1965).

Kanta and Maheshwari (1963) also tried to bring about fertilization of ovules in vitro. The un-pollinated ovules of Papaver somniferum were sown on an agar medium in a test tube. The pollen grains were dusted over the implanted ovules. The pollen grains germinated, and the pollen tubes grew rapidly and covered the ovules. Successful fertilization occurred in many ovules.


The Angiosperms

· Discuss the relationship of Charophytes to land plants.

· Explain the alternation of generation life cycle seen in plants and identify the dominant generation in different types of plants.

· Discuss the major groups of plants: nonvascular plants, seedless vascular plants, and seed plants.

· Describe the difference between vascular and nonvascular plants.

· Discuss the difference between seedless vascular plants and vascular seed plants.

· List adaptations seen in seed plants.

· List the different groups of seed plantsGymnosperms: Cycadophyta, Ginkgophyta, Gnetophyta, Coniferophyta, and

Angiosperms: Magnoliophyta.

· In regard to seed plants – angiosperms (“covered seeds”)

o Identify the structures of a flower and their functions.

o Compare eudicot (dicot) and monocot flowers.

o Describe the life cycle of a typical flowering plants.

o Identify which part of a flower contains pollen and which contain ovules, and explain where one would find the male and female gametophytes and seeds.

o Identify some of the common basal Angiosperm (neither monocot nor dicots) that grow in West Virginia

This Lab Guide covers:

I. Introduction to Angiosperms

II. Structure of flowering plants (roots, stems, and leaves)

III. Reproduction of flowering plants (flowers, fruits, and seeds)

The seed plants are divided into two major groups: The non-flowering Gymnosperms and the flowering Angiosperms. Study them in your textbook, handouts, slides, lab manual or atlas (whatever applies). Angiosperms have a sporic life cycle (“Meiosis produces spores”). They are the dominant plants seen today. This is likely because of some key adaptations that include producing flowers and fruits. Flowers contain the male and female reproductive structures, either on the same or different flowers depending on the species.

Many flowering plants have symbiotic relationships with animals. The animals might help the plants in form of pollinators or seed disperser. As pollinators, animals often receive food or some benefit when they visit flowers. Pollinators aid in reproduction by carrying pollen to different plants, which aids in the genetic diversity of the plant species.

Fruits are an important tool for seed dispersion. Fruits develop from the plant’s ovary located in the flower after fertilization. Flowering plants undergo double fertilization (see textbook). Upon pollination, two sperm will fertilize two different cells. One cell will develop into the embryo, while the other cell will develop into a structure that will nourish the embryo called the endosperm.

While there are several different groups of angiosperms, we will concentrate on representatives of the Basal Angiosperms native to WV, as well as Euicotyledones (eudicots), and Monocotyledons (monocots)

Examples for the Basal Angiosperms are magnolias, nutmeg, bay laurel, cinnamon, avocado, black pepper, tulip tree, pawpaw, spicebush, and sassafras. The last four are native to West Virginia.

The basal angiosperms are a broad group of the most primitive flowering plants. They belong neither to the monocotyledons nor to the eudicotyledons. However, the dicots include the basal angiosperms as well as the eudicots. You will later learn the typical characteristics of monocts (short for monocotyledons) and eudicots (short for eudicotyledons) in regards to their seedling leaves, leaf venation, vascular bundle as well as flower petal arrangements. The basal angiosperms do not place appropriately into either category.

The spicebush Lindera benzoin and sassafras Sassafras albidum both belong to the Laurel Family (Order Laurales). The vegetative parts of Lauraceae are extremely fragrant. Sassafras has a pleasant soapy odor.

The pawpaw Asimina triloba and the tulip tree Liriodendron tulipifera belong to the Magnolia Family (Order Magnoliales)

<- Spicebush Lindera benzoin with berries in the Fall, Parkersburg, WV. Only female pants produce these spicy, edible berries. West Virginia, USA (Never eat any berries that you have not identified 100%!)

Fun Fact: Not many people outside of the Appalachian region and Ohio have heard of PawPaw. A few years ago (maybe 2014?) the New York Times published typical Thanksgiving dinner dishes around the USA. For West Virginia they listed PawPaw Pie pudding. None of my friends and neighbors native to WV had ever heard of it. To add insult to injury, the accompanying illustration showed a papaya! This is further proof that a solid science education is important.

The Angiosperms (Magnoliophyta) are the flowering plants with covered seeds.

In the following we are going to study the two major groups of flowering plants the monocots and eudicots (dicots). Several characteristics are used to distinguish plants in these two groups. See more here: Monocot and Dicot Comparison

  1. Number of seedling/germ leaves
  2. Leaf venation
  3. Arrangement of vascular bundles
  4. Flower parts in multiples of 4 or 5 OR multiples of 3.
  5. Root growth

No one characteristic should be used alone to identify a plant, but multiple characteristics should be used to determine if a plants belongs to one or the group. Use the information provided in the lab, your textbook, and atlas to fill in table below the distinguishing characteristics of monocots and dicots.

Monocots vs. Dicot Comparison of Characteristics

Fill in this table the information you gather during this exercise. Include both anatomical ( in this case: microscopic) and morphological (growth type) characteristics. Use your textbook or atlas to find anything you could not find here.

Zea mays: Corn seed embryo drawing

Bean seed/embryo: 1. Epicotyl (plume), 2. Cotyledon (one of two), 3. Hypocotyl (in this case also the side of shoot apical meristem), 4. Radicle, 5. side of root apical meristem, 6. Integument/Seed coat.

The functions of roots are to anchor the plant into the soil and to absorb water and minerals. They often store nutrients as seen in turnips, sweet potatoes, carrots, and radish. The majority of the root system is hidden from our view and develops underground. However, the root system is usually extensive and can equal the plant mass above ground.

Of course there are exceptions! The dodder, Cuscuta sp., a parasitic plant without leaves, inserts its roots to her host and "steals" essential nutrients. Dodder plants are native to West Virginia and its surrounding states.

Ficus aureus, the Florida Strangler Fig, starts out as an epiphyte. The seeds germinate on the supporting tree and the fig's roots grow towards the ground. From there on, it becomes an independent tree. However, accidents might happen and the figs might strangle the former host. Other trees can enclose whole buildings!

Often plants form complex relationships (symbiosis), such as Mycorrhiza with fungi or as seen in many legumes, form root nodules with nitrogen fixing bacteria to help them take up nutrients.

Roots play an important role in the ecosystem, by keeping the soil together. This is particular important on artificial hill slopes after constructions and areas that are prone to erosion such as rivers banks and shores.

Root Diversity

1. Observe roots in your textbook and atlas.

2. Observe different types of roots in the lab. Be able to identify:

      • Fibrous root system – all roots are about the same size.
      • Taproot – there is one main root with many smaller branch roots.
      • Adventitious roots – roots that develop from non-root tissue like a stem. Prop roots are a type of adventitious roots. Aerial roots, like those that attach ivy to walls, are also adventitious roots.

      Various Roots Types in Angiosperms: On the left side, an eudicot, middle and right are monocots.

      In your textbook or atlas, review the monocot and eudicot root cross-sections. In the pictures below, identify the epidermis, endodermis, cortex, phloem, and xylem. Know that only roots have an endodermis!

      • Epidermis cells cover plant leaves, flowers, roots, and stems—usually, a single layer. In roots, the epidermis helps to absorb water and mineral nutrients. In other plant parts, it protects against water loss, regulates gas exchange, and secretes metabolic compounds.
      • Cortex is the outermost layer of a stem or root. It connects on the outside to the epidermis and on the inside to the endodermis.
      • The endodermis is the central, innermost layer of cortex in some land plants. It covers the stele and the Casparian strip (an outer ring of endodermal cells surrounds it.
      • The Casparian strip is a band of cells that contain hydrophobic substances (for waterproofing) joining the walls of endodermal cells. The Casparian strip forces water to move by osmosis through the cell membranes instead of flowing through the porous cell walls.
      • The pericycle is a cylinder of parenchyma or sclerenchyma cells. It lies inside the endodermis and is the outermost part of the stele of plants.
      • The stele is the central part of the root.
      • Pericycle cells actively transport minerals into the interior of the vascular cylinder, maintaining a concentration gradient that keeps the minerals diffusing inward and water following by osmosis.

      When a seed germinates, the first structure to emerge is a root. The radicle usually develops into a single taproot with small lateral branches as seen in the picture to your right.

      The root structure of of monocotyledons and eudicotyledons is different in many ways:

      In monocot roots the xylem and phloem form a circle. The endodermis in the picture on the left forms a distinct ring around the pericycle.

      The eudicot root contains the xylem in the middle and the phloem surrounds it. The pith is usually small and encircled by the endodermis.

      Tissue at the tip of a root contains specialized cells that can become any tissue in the plant. This tissue is called meristematic tissue, and it allows the plant to grow longer for its entire life. Meristematic tissue at the tip of a plant is known as an apical meristem. Apical meristem can also be found at the tip of shoots actively growing upward toward the sun. Cells in these regions are actively undergoing mitosis. The growth that results in vertical growth or an increase in length is known as primary growth. Protective root cap cells cover the tip of the root and protect the meristematic tissue. Above the meristematic tissue, cells elongate and increase in size in a region known as the Elongation Region. Cell differentiation occurs in the Maturation Region as newly formed cells become specific types of tissue, such as ground tissue, dermal tissue, or vascular tissue.

      1. Compare to atlas what you learned here to your atlas or textbook.

      Observe a cross-section of a monocot root slide, such as a Smilax sp. and a dicot root such as Ranunculus sp. or see the microscopic picture on this website. Be able to identify the following structures:

      a. Epidermis

      c. Endodermis

      Online Students: Take a picture at 100 x of your root slides and identify and label the structures above. Clearly state if the specimen is a monocot or dicot root. Assembly your pictures in one PDF document and save as root_name.pdf

      2. Observe a root tip in the lab/lab guide. Be able to identify the following regions:

      a. Root cap

      b. Meristematic region – apical meristem

      c. Elongation region

      d. Maturation region

      e. Root hairs

      Stems usually grow upwards so that their leaves are exposed to sunlight. Stems also contain the conducting vessels xylem and phloem. Most stems are found above ground where they provide support for leaves and flowers. Vascular tissue that runs through stems provides vital water, minerals, and nutrients between the roots, leaves, and flowers.

      Plants that have nonwoody stems are referred to as herbaceous plants, while plants that contain wood are considered to be woody plants. Herbaceous stems and woody stems undergo primary growth (vertical growth) at the tip of branches called shoot tips. Meristematic tissue undergoes cell division to allow for continued vertical growth. Like in the root tip, these cells are located in apical meristem at the shoot tip.

      Woody plants may also undergo secondary growth (lateral growth). Meristematic tissue increases the girth or diameter of the stem, which adds additional support to the plant as it grows vertically.

      Stem Diversity

      While most stems are found above ground, supporting leaves and flowers, some stems have evolved to provide additional functions for the plant, such as storage of photosynthetic products. Observe some examples of stem diversity in the lab and you manual/textbook.

      Plants that have woody stems are often dicots. They experience both primary growth (vertical growth=increase in length) and secondary growth (horizontal growth=increase in girth). This is accomplishment by the cell division activity in the apical meristem at the tip of a branch within a terminal bud. Meristematic tissue is also found in the vascular cambium and is responsible for secondary growth. Each year new xylem and phloem are made by the vascular cambium and is called secondary xylem and phloem. Wood is produced by the buildup of secondary xylem year after year.

      Study the structures of an eudicotyledon and monoctyledon stem as seen below and compare it with your textbook and/or Biology Atlas.

      If you want to determine the age of a tree branch or trunk, you can count the annual rings to calculate the age of the wood. With a small twig, this procedure might not be feasible.

      To determine the age of a winter twig, find the terminal bud and then find the previous terminal bud scar. That represents one year of growth. Continue counting the terminal bud scars until you get to the base of the twig, adding a year with each terminal bud scar. The last terminal bud scar and the base of the twig also represent one year of growth. The diameter or the length of a branch or twig does not necessarily indicate its age.

      See more instruction on how to determine the age of a twig here:

      Shoot Tip and Variations of Stems

      1. See Figure in your atlas/textbook of a longitudinal section of a stem tip.

      2. Observe slide of a longitudinal section of a stem tip (if available). Be able to identify:

      a. Apical meristem

      3. Observe stem diversity in the lab. Be able to identify and explain the function of the following stem types:

      a. Stolon or Runners – These are horizontal stems that run along the ground and produce adventitious roots and new shoots at nodes. (Example – strawberry runner)

      b. Rhizomes – below ground horizontal stems. These function as food storage for the plant. New plants can develop from a piece of rhizome. Adventitious roots extend from these structures. (Example – iris)

      c. Tubers – develop from below ground rhizomes and is below ground stem that serves as a site of food storage. Each eye on the potato can develop into a new plant. (Example – potato, but NOT sweet potato)

      d. Corm – a below ground vertical stem, which functions in food storage and has thin papery leaves but is solid when cut in half. It can produce shoots that can develop into above ground stems that produce normal leaves and flowers. It also can produce adventitious roots. (Example – gladiolus)

      e. Bulb – a below ground vertical stem, which functions in food storage and also is made up of layers of modified leaves. Unlike corms, bulbs are not solid structures, but when cut in half, reveal multiple layers of modified leaves. It can produce shoots that can develop into above ground stems that produce normal leaves and flowers. It also can produce adventitious roots. (Example – onion)

      f. Tendrils – above ground stems that can wrap around other structures in order to secure the plant as it grows. It is used for support and attachment. (Example – peas, green beans, ivy)

      g. Cladode – specialized stem that often is leaf-like in appearance may be flattened like a leaf. Photosynthesis occurs in these structures. (Ex. Christmas cactus [Schlumbergera sp.], asparagus)

      2. Observe a cross-section of a tree. Be able to identify:

      a. Bark – consists of a protective outer layer called cork, followed by the cortex that stores photosynthetic materials, and phloem, which transports photosynthetic material in the plant. The vascular cambium is found at the inside edge of the bark.

      b. Annual rings – part of what is called wood. It consists of secondary xylem layers (xylem from previous years) and primary xylem.

      3. Observe a microscope slide of a cross-section of a woody stem. Be able to identify the following structures:

          • Periderm
          • Cork Cambium
          • (secondary) Phloem
          • Vascular Cambium
          • Secondary Xylem: Spring and Summer Wood.

          1. Observe winter twigs in the lab/this LM. To determine the age of a winter twig, find the terminal bud and then find the previous terminal bud scar. That represents one year of growth. Continue counting the terminal bud scars until you get to the base of the twig, adding a year with each terminal bud scar. The last terminal bud scar and the base of the twig also represent one year of growth. See atlas and lab material.

          Be able to identify the following structures:

          a. Terminal bud – This will be the site of new, primary growth. New tissues will be produced here, including vascular bundles and leaves.

          b. Axillary buds – new branch growth can occur here.

          c. Bud scale – leaf-like scales that cover and protect the tip of the branch during dormancy. These will fall off when new growth begins. The scars left by this will produce a terminal bud scar.

          d. Terminal bud scar – These scars encircle the branch and are the site of where a terminal bud was once located in previous years. Terminal bud scars can be used to age a branch.

          e. Leaf scar – where a leaf was attached to a stem.

          f. Lenticel – sites in the stem that allow for gas exchange.

          g. Vascular bundle scars – are in the leaf scar and indicate where vascular tissue was once attached to vascular tissue in a leaf.

          h. Node – place where leaf scars and vascular bundle scars are found.

          i. Internode – space between nodes.

          2. Try to age winter twigs either in the lab or from twigs you collected.


          Angiosperm

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          angiosperm, any of about 300,000 species of flowering plants, the largest and most diverse group within the kingdom Plantae. Angiosperms represent approximately 80 percent of all the known green plants now living. The angiosperms are vascular seed plants in which the ovule (egg) is fertilized and develops into a seed in an enclosed hollow ovary. The ovary itself is usually enclosed in a flower, that part of the angiospermous plant that contains the male or female reproductive organs or both. Fruits are derived from the maturing floral organs of the angiospermous plant and are therefore characteristic of angiosperms. By contrast, in gymnosperms (e.g., conifers and cycads), the other large group of vascular seed plants, the seeds do not develop enclosed within an ovary but are usually borne exposed on the surfaces of reproductive structures, such as cones.

          What are angiosperms?

          Angiosperms are plants that produce flowers and bear their seeds in fruits. They are the largest and most diverse group within the kingdom Plantae, with about 300,000 species. Angiosperms represent approximately 80 percent of all known living green plants. Examples range from the common dandelion and grasses to the ancient magnolias and highly evolved orchids. Angiosperms also comprise the vast majority of all plant foods we eat, including grains, beans, fruits, vegetables, and most nuts.

          How are angiosperms different than gymnosperms?

          The key difference between angiosperms and gymnosperms is how their seeds are developed. The seeds of angiosperms develop in the ovaries of flowers and are surrounded by a protective fruit. Gymnosperm seeds are usually formed in unisexual cones, known as strobili, and the plants lack fruits and flowers. Additionally, all but the most ancient angiosperms contain conducting tissues known as vessels, while gymnosperms (with the exception of Gnetum) do not. Angiosperms have greater diversity in their growth habits and ecological roles than gymnosperms.

          How are angiosperms and gymnosperms similar?

          As vascular plants, both groups contain xylem and phloem. With the exception of a very few species of angiosperms (e.g., obligate parasites and mycoheterotrophs), both groups rely on photosynthesis for energy. Angiosperms and gymnosperms both utilize seeds as the primary means of reproduction, and both use pollen to facilitate fertilization. Gymnosperms and angiosperms have a life cycle that involves the alternation of generations, and both have a reduced gametophyte stage.

          Unlike such nonvascular plants as the bryophytes, in which all cells in the plant body participate in every function necessary to support, nourish, and extend the plant body (e.g., nutrition, photosynthesis, and cell division), angiosperms have evolved specialized cells and tissues that carry out these functions and have further evolved specialized vascular tissues (xylem and phloem) that translocate the water and nutrients to all areas of the plant body. The specialization of the plant body, which has evolved as an adaptation to a principally terrestrial habitat, includes extensive root systems that anchor the plant and absorb water and minerals from the soil a stem that supports the growing plant body and leaves, which are the principal sites of photosynthesis for most angiospermous plants. Another significant evolutionary advancement over the nonvascular and the more primitive vascular plants is the presence of localized regions for plant growth, called meristems and cambia, which extend the length and width of the plant body, respectively. Except under certain conditions, these regions are the only areas in which mitotic cell division takes place in the plant body, although cell differentiation continues to occur over the life of the plant.

          The angiosperms dominate Earth’s surface and vegetation in more environments, particularly terrestrial habitats, than any other group of plants. As a result, angiosperms are the most important ultimate source of food for birds and mammals, including humans. In addition, the flowering plants are the most economically important group of green plants, serving as a source of pharmaceuticals, fibre products, timber, ornamentals, and other commercial products.

          Although the taxonomy of the angiosperms is still incompletely known, the latest classification system incorporates a large body of comparative data derived from studies of DNA sequences. It is known as the Angiosperm Phylogeny Group IV (APG IV) botanical classification system. The angiosperms came to be considered a group at the division level (comparable to the phylum level in animal classification systems) called Anthophyta, though the APG system recognizes only informal groups above the level of order.

          Throughout this article the orders or families are given, usually parenthetically, following the vernacular or scientific name of a plant. Following taxonomic conventions, genera and species are italicized. The higher taxa are readily identified by their suffixes: families end in -aceae and orders in -ales.

          For a comparison of angiosperms with the other major groups of plants, see plant, bryophyte, fern, lower vascular plant, and gymnosperm.


          Reproduction in angiosperms.

          This lesson begins with a short screencast activity to help students draw the structure of an idealised animal-pollinated flower. In nature the shape of flowers is very varied and many flowers have a shape which complements the adaptations of a mutualistic pollinator. This idea makes a nice "dissection" Comparisons of different flowers helps students to understand the individual adaptations of flowers and their pollinator.

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          What are Angiosperms?

          Flowering plants are called as angiosperms. The flowering plants are the most dominant vascular plants that are found in the fauna all around the world. The pleasing and attractive colours of their flowers certainly add much more colour and brighten the landscape of any place.

          Due to the presence of flowers and enclosed seeds, they are called the phanerogams. Scientifically speaking, in these plants, the seeds are enclosed, with the ovules present in a hollow ovary.

          Browse more Topics under Plant Kingdom

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          Features of Angiosperms

          All angiosperm plants have the characteristic vascular bundle with the xylem and phloem tissues for conduction of water, minerals, and nutrients. The plant body is well differentiated with a well-developed root system, shoot system and leaves. Specialised structures called as the flowers are present. Within these flowers, the male and female gametes develop. After fertilization, when these flowers mature, fruits are formed which have the seeds within them.

          Angiosperms can be found in varied habitats and can come in a different range of sizes. Wolfie is an angiosperm that is microscopic whereas the Australian mountain ash tree is about 100 meters tall. The diversity that the angiosperms display is very wide. There are many plants that are tall woody trees, shrubs, and even herbaceous plants. These plants also have many adaptations in the roots, stems and leaves depending on the habitat that they grow in.


          Cycads are considered endangered species and their trade is severely restricted. Customs officials stop suspected smugglers, who claim that the plants in their possession are palm trees and not cycads. How would a botanist distinguish between the two types of plants?

          The resemblance between cycads and palm trees is only superficial. Cycads are gymnosperms and do not bear flowers or fruit. Unlike palms, cycads produce cones large, female cones that produce naked seeds, and smaller male cones on separate plants.

          What are the two structures that allow angiosperms to be the dominant form of plant life in most terrestrial ecosystems?

          Angiosperms are successful because of flowers and fruit. These structures protect reproduction from variability in the environment.


          Watch the video: Plant Reproduction in Angiosperms (August 2022).