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Sexual Reproduction in Flowering Plants: Sexual reproduction is the formation of new individuals through the meiotic gamete formation and their subsequent fusion during fertilization. It is also called amphimixis.
The flower is a specialized condensed shoot meant for carrying out sexual reproduction. It bears floral leaves and gives rise to seeds and fruits.
A typical flower has 4 whorls of floral leaves viz.,
- sepals (calyx)
- petals (corolla)
- stamens (androecium)
- carpels (gynoecium)
Stamens and carpels respectively represent male and female reproductive organs of flowers.
MALE REPRODUCTIVE ORGAN
Androecium consists of stamens as its units. The stamens or microsporophylls are regarded as the male reproductive organs of the flower.
Each stamen consists of three parts – Filament, anther, and connective.
The filament is the slender stalk of the stamen, and the anther is the expanded head, borne by the filament.
Each anther consists usually of two lobes connected together by a sort of midrib known as the connective.
Anther consists of 4 pollen sacs or microsporangia (two in each lobe). The microsporangial wall has four types of layers:
Epidermis: One cell thick and protective in function.
Endothecium: Single layered, cells have cellulose thickening with a little pectin and lignin in some cases. It helps in anther dehiscence.
Middle layers: 1-3 layers; they degenerate during the maturity of the anther.
Tapetum: Tapetal cells are nutritive. By these cells, the Ubisch bodies are produced which help in the ornamentation of microspore wall.
The process of formation of microspores from sporogenous tissue is known as microsporogenesis.
The sporogenous cells may directly function as microspore mother cells (also called pollen mother cells or PMCs).
As the anther matures and dehydrates, it dehisces by slits and pores to liberate the pollen grains.
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Pollen grain (microspore) is the first cell of the male gametophyte.
Pollen grains are released from the anthers either at 2 – celled stage or at 3 – celled stage.
Wall or covering of pollen grain is called sporoderm. It has two layers, outer exine, and inner intine. The outer layer exine is thick and sculptured or smooth. It is made up of sporopollenin which is resistant to chemical and biological decomposition so the pollen wall is preserved for long periods in fossil deposits.
At certain places, the exine remains thin. These thin areas are known as germ pores when they are circular in outline and germinal furrows when they are elongated. The intine is thin and elastic. It emerges out as the pollen tube from the germ pores during germination.
Development of male gametophyte is precocious, that is, it begins inside the microsporangium or pollen sac.
The size of the nucleus increases and protoplast then divides mitotically to produce a bigger vegetative or tube cell and a smaller generative cell.
The tube cell has a vacuolate cytoplasm which is rich in food reserve cell organelles and large and irregular nucleus.
The generative cell is small, spindle-shaped to spherical-shaped, with thin dense cytoplasm and a prominent nucleus.
Male gametes are formed by a mitotic division in a generative cell.
FEMALE REPRODUCTIVE ORGAN
The gynoecium is a female reproductive whorl of the flower. It is composed of one or more carpels or pistils.
Each pistil consists of three parts, i.e., stigma, style, and ovary.
A pistil has a terminal receptive disc-like stigma, a stalk-like style and a basal swollen ovule bearing part called the ovary. Inside the ovary are the ovarian cavity (with one or more locules) and parenchymatous cushion called the placenta, from which arise ovules.
An ovule or integument megasporangium develops from the base or the inner surface of the ovary. It is small, generally an oval structure and consists chiefly of a central body of tissue, the nucellus, and one or two integuments.
Each ovule is attached to the placenta by a small stalk called the funiculus.
The place of attachment of the funiculus with the main body of the ovule is called the hilum.
In a typical (anatropous) ovule, the funiculus is fused with the body of the ovule lengthwise beyond the hilum. It gives rise to a longitudinal ridge called raphe. Funiculus contains a vascular strand for the supply of nourishment to the ovule.
The ovule contains a mass of thin-walled parenchymatous cells called nucellus. The nucellus is protected by one or two multicellular coats called integuments. The basal portion of the nucellus from where the integuments appear is called chalaza. A small opening isle at the apex of integuments known as micropyle.
Embryo sac or female gametophyte is embedded in the micropylar half of the nucellus. This makes the most important part of the mature ovule. It is the embryo sac, which bears the embryo later on.
Formation of megaspore (n) from megaspore mother cell (2n) by meiosis inside the ovule is called megasporogenesis.
Only one megaspore mother cell remains functional and the other 3 degenerates. This functional megaspore develops into a female gametophyte. This method of embryo sac formation from a single megaspore is called monosporic development.
The functional megaspore is the first cell of the female gametophyte. The nucleus of megaspore divides mitotically to form 8 nuclei and give rise to embryo sac or female gametophyte. This process is called megagametogenesis.
Out of 8 nuclei, 4 nuclei are arranged at each pole. One nucleus from each pole migrates to the center to form two polar nuclei which further fuse to form a diploid or secondary nucleus.
Three nuclei at the base of embryo sac form antipodal cells. The remaining 3 nuclei at the micropylar end get surrounded by cytoplasm and are called egg apparatus. The egg apparatus consists of one larger middle cell known as egg or oosphere and the remaining two cells are called as synergids or help cells.
Thus a typical angiosperm embryo sac, at maturity, though 8-nucleate is 7 celled with 3 micropylar, 3 chalazal and one central cell.
POLLINATION
Pollination refers to the transfer of pollen grains from the anther to the stigma of a flower. It is of two types: self-pollination and cross-pollination.
Agents of pollination
The transfer of pollen grains from plant to plant in cross-pollination takes place through biotic agencies (living agencies) or through abiotic agencies (non-living agencies).
Pollen – Pistil Interaction
Pollen – pistil interaction is a safety measure to ensure that illegitimate crossing does not occur.
It checks the promotion or inhibition of pollen growth. Pollen grains of a number of plants may settle over a stigma but only the right pollen belonging to the same species would germinate while others fail to do so.
The compatibility and incompatibility of the pollen-pistil is determined by special proteins. The compatible pollens are able to absorb water and nutrients from the surface of the stigma. They germinate and produce pollen tubes. Pollen tubes grow into the style. Their growth and path through the style are also determined by specific chemicals.
Significance of pollination
Pollination is a means of taking the male gametophyte for its growth near the female gametophyte.
Pollen-pistil interaction determines the suitability of pollen for carrying out the process of sexual reproduction.
It has freed the seed plants from the dependence on external water during fertilization.
It can be manipulated to produce pure lines as well as desired varieties.
Fertilization
The fusion of male and female gamete is called fertilization.
The fertilization involving the formation of the pollen tube as in seed plants is called siphonogamy.
On reaching the ovary, the pollen tube grows towards one of the ovules.
The pollen tube enters the ovule either through its micropyle (porogamy e.g., lily) or chalaza (chalazogamy e.g., Betula) or integuments (mesogamy e.g., Cucurbita). Irrespective of the place of entry into ovules, the pollen tube enters the embryo sac through the micropylar region.
Inside the embryo sac, one male gamete fuses with the egg to form the zygote (2n), the process is known as syngamy or generative fertilization. The second male gamete fuses with 2 polar nuclei or secondary nucleus to form triploid primary endosperm nucleus, the process is known as a triple fusion or vegetative fertilization.
The zygote develops into an embryo and the primary endosperm nucleus develops into endosperm.
The occurrence of syngamy and triple fusion simultaneously in angiosperms is called double fertilization. Double fertilization was Thirst reported by S.G. Nawaschin (1898) in Fritillaria and Lilium.
Post-fertilization: STRUCTURE AND EVENTS
The events of endosperm and embryo development, maturation of ovule(s) into seeds and ovary into fruit, are collectively termed as post-fertilization events.
Endosperm
As a consequence of triple fusion in which a male gamete fuses with the secondary nucleus, a triploid structure called primary endosperm nucleus (PEN) is formed that divides by mitotic divisions and forms a mass of nutritive cells called the endosperm.
Endosperm development precedes embryo development. Endosperm accumulates food reserves and functions as the nutritive tissue for the developing embryo.
Embryo
After fertilization, the fertilized egg is called zygote or oospore which develops into an embryo.
This process of development of the embryo is called embryogenesis.
The oospore before entering the process of embryogenesis undergoes a period of rest which may vary from a few hours to a few months.
The embryo develops at the micropylar end of the embryo sac where the zygote is situated. Most zygotes divide only after a certain amount of endosperm is formed.
A typical dicotyledonous embryo consists of an embryonal axis and two cotyledons. The part of the embryonal axis above the level of cotyledons is called epicotyl. It terminates with the stem tip, called plumule (future shoot). The part below the level of cotyledons is called hypocotyl which terminates in the root tip called radicle (future root). The root tip is covered with a root cap (calyptra).
In monocotyledons, there is only one terminal cotyledon situated towards the lateral side of the embryonal axis called scutellum (shield-shaped) e.g., grass family. At its lower end, the embxyonal axis has radicle and root cap enclosed in an undifferentiated sheath called coleorhiza.
The part of the axis above scutellum attachment is called epicotyl. It has shoot apex and few leaf primordia enclosed in a hollow foliar structure called coleoptile. The early development of dicot and monocot embryos is similar up to the globular stage.
SEED
Ripened ovules are known as seeds. A true seed is defined as a fertilized mature ovule that possesses an embryonic plant, stored food (sometimes absent) and a protective coat or coats.
Integuments of ovule harden as tough protective seed coat. Outer integument forms testa and tegmen develop from the inner integument.
Funiculus (stalk of ovule) forms stalk of the seed. Ultimately, stalk withers and leaves a minute scar called hilum. Antipodals, synergids disorganize and nucellus may disappear or persist as perisperm.
FRUITS
The fruit is a seed containing part of a plant that develops from a fertilized ovary and often from other tissues that surround it.
A pseudocarp or accessory fruit or false fruit is a fruit whose _esh is derived from the ovary, along with some other floral parts like the thalamus, e.g., apple, strawberries.
A eucarp or true fruit is a fruit that is developed only from the ovary after fertilization, e.g., mango.
The wall covering the fruit is called the pericarp. It consists of three parts:
Epicarp called the skin of fruits.
Mesocarp, middle fleshy and pulpy part.
Endocarp, the innermost portion that surrounds the seeds.
Fruits can be simple fruit (develops from a monocarpellary ovary), aggregate fruit (develops from a multi carpellary apocarpous ovary) and composite fruits (develops from a complete inflorescence).
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