02. Sexual Reproduction in Flowering Plants
02. Sexual Reproduction in Flowering Plants
- The development of a flower involves many hormonal and structural changes.
- Flower buds, and then the flowers, are borne by inflorescences.
- The reproductive parts of a plant are its flowers.
- The male reproductive part—the androecium—and the female reproductive part—the gynoecium—develop in the flowers.
- The androecium is made up of whorls of stamen.
- The stamen has two parts—the filament and the anther. The filament is a long and slender stalk, whereas the anther is a bilobed structure.
- The filament is joined to either the thalamus or to the petal.
- An anther consists of two dithecous (two thecae) lobes.
- Each theca is separated from the other by a longitudinal groove, which runs lengthwise.
- In the corner of each theca, two microsporangia are located. The microsporangia develop into pollen sacs, which contain pollen grains.
Structure of microsporangium
- The microsporangium has four walls around it—epidermis, endothecium, middle layers, tapetum.
- The function of the outer three layers is that of protection and dehiscence of the anther to expel pollen grains. The function of the tapetum is to nourish the pollen grains.
- In the immature anther, the center of each microsporangium is formed of sporogeneous tissue.
- It refers to the process of development of microspore from pollen mother cells (PMC).
- In the anther, sporogeneous tissue undergoes meiosis and forms a microspore tetrad.
- Each cell is a potential pollen or PMC because each cell of the sporogeneous tissue can give rise to a tetrad.
- Microspores separate from each other as the anther matures and these microspores become pollen grains.
- Are representative of the male gamete, have a two-layered wall, and are spherical.
- Exine (outer) layer: Composed of sporopollenin, which makes it hard. It is resistant to high temperature, alkaline, acid, and enzymes.
- Intine (inner) layer: Composed of pectin and cellulose, it is a thin and continuous layer.
- Mature pollen grains consist of two types of cells:
- Large, vegetative cell with an irregular nucleus and food reserves
- Small, generative cell which floats in the cytoplasm of the vegetative cell
- Pollen grains are expelled at the two-cell stage in 60 percent of angiosperms, whereas in the rest, a three-cell stage or two male gametes are formed by the process of mitosis.
- After pollen grains are expelled, they may remain viable between 30 minutes and a few months. Viability depends on temperature and humidity.
Gynoecium and the Formation of the Female Gametophyte
- The gynoecium is representative of the female reproductive part of a flower.
- The gynoecium may have one pistil (monocarpellary) or many pistils (multicarpellary). Multicarpellary flowers may have fused pistils (syncarpous) or free pistils (apocarpous).
- A pistil has three parts:
- Stigma, which accepts the pollen grains.
- Style, which is the slender, elongated part located below the stigma
- Ovary, which is the swollen basal portion that houses the placenta. The placenta is held inside the ovarian locule or cavity. The placenta carries the ovules or megasporangia.
- The funicle attaches the ovule to the placenta. The hilum is the junction at which the ovule and the funicle meet.
- The protective layers (1-2) of the ovule are known as integuments, which cover the ovule entirely except for a small opening known as micropyle.
- The basal portion of the ovule is represented by the chalaza, which lies at the opposite end of the micropyle.
- Within the integuments, the nucellus is present which contains reserve food. Within the nucellus, the female gametophyte or embryo sac is located.
- Megasporogenesis is the process by which the megaspore mother cell (MMC) transforms into megaspores.
- The MMC is a large cell, which contains a prominent nucleus within dense cytoplasm. Meiosis converts the MMC into four megaspores.
- Usually, only one megaspore remains functional while the rest degenerate.
- Via monosporic development, this megaspore becomes the female gametophyte.
- The nucleus of the megaspore divides via mitosis to give rise to two nuclei. Each nucleus moves to the opposite ends of the cell, forming a two-nucleate embryo sac. Mitosis occurs two more times, giving rise to four-nucleate and eight-nucleate embryo sacs.
- After the eight-nucleate embryo sac is formed, cell walls develop, and the typical embryo sac or female gametophyte is organized.
- The cell wall surrounds six nuclei. The two remaining nuclei, called polar nuclei, stay in the large central cell, below the egg apparatus.
- The egg apparatus comprises two synergids and one egg cell. The egg apparatus is formed by three nuclei (out of the six nuclei), which are located at the micropylar end.
- The synergids at the micropylar end have special thickenings and are together called the filiform apparatus. These thickenings lead the pollen tubes into the synergids.
- Antipodal cells comprise are formed of the three cells located at the chalazal end.
- When fully mature, a typical angiosperm female gamete has seven cells and eight nuclei.
- When pollen grains get transferred from the anther to the stigma, the process is called pollination.
- Pollination can be variously categorized based on the source of pollen.
- Autogamy: When pollen grains are transferred from the anther to the stigma of the same flower. For successful autogamy, the anther and stigma should be located close to each other and pollen release and stigma receptivity should be synchronized. Plants like Oxalis and Viola have two types of flowers—chasmogamous (exposed anther and stigma) and cleistogamous (closed—only autogamy happens).
- Geitonogamy: When pollen grains are transferred from the anther of one flower to the stigma of another flower within the same plant. The process is genetically similar to autogamy, however pollinating agents are required.
- Xenogamy: When pollen grains are transferred from the anther of one flower to the stigma of another flower in a different plant. Genetically different pollen is brought to the plant.
Agents of Pollination
Various agents—air, water (abiotic), and animals (biotic)—are used by plants for pollination.
Pollination by wind
- Most common type of abiotic pollination
- Plants using this agent have well-exposed anthers and feathery, large stigmas.
- Pollen is light and non-sticky and can be easily carried by the wind.
- Flowers of such plants often have a single ovule inside the ovary and several flowers packed in the inflorescence.
- Commonly seen in grass.
Pollination by water
- Seen in plants like water lily and water hyacinth, where flowers rise above the water level and are pollinated by insects.
- Pollen grains are long and ribbon-like and have a mucilaginous covering to protect against wetting.
- This agent is rarely observed in flowering plants, except for Hydrilla and Vallisneria.
Pollination by animals
- Most flowering plants take the help of bees, wasps, and butterflies to undertake pollination.
- Most flowers are large, colorful, fragrant, and contain nectar (floral rewards) to attract animal pollinators.
- Other floral rewards are a secure place to lay eggs e.g. Amorphophallus—the tallest flower.
- The plant Yucca and its pollinator moth have a symbiotic relationship. The moth lays its eggs in the locule of the ovary and, in return, the plant gets pollinated.
- Pollen grains are sticky and attach to the body of the pollinator.
- Inbreeding depression is caused by repeated self-pollination.
- To prevent self-pollination, plants have devised several ways.
- Autogamy is prevented by:
- No synchronization of pollen release and stigma receptivity
- Production of unisexual flowers
- Change in positioning of anther and stigma
- Autogamy and geitonogamy are prevented by:
- Presence of male and female flowers on different plants—each plant is either male or female (dioecy)
- Several species of papaya use this mechanism.
- Autogamy is prevented by:
- Compatible pollen grains are not always transferred by pollination.
- The pistil can recognize the correct type of pollen grain to encourage post-pollination events.
- The pistil prevents pollen germination if the pollen grain is not of the correct type.
- Chemical constituents of the pistil and the pollen bring about this interaction.
- Pollen-pistil interaction is a dynamic process, wherein the pistil recognizes the pollen and then either promotes or inhibits it.
- The pollen tube travels to the ovary and enters the ovule through the micropyle. It locates the synergids through the filiform apparatus. Thus, the pollen tube grows like this.
Artificial Hybridization and Double Fertilization
- This method makes crop yield better.
- The stigma is protected from the wrong kinds of pollen by:
- Emasculation: If the female parent has bisexual flowers, the anther is taken away from the bud.
- Bagging: The emasculated flower is covered with a bag which prevents the contamination of stigma by unwanted pollen.
- When the stigma of the bagged flower turns receptive, the bag is removed, and suitable collected pollen is dusted on it. Then, the flower is rebagged.
- If the female parent is unisexual, emasculation is not required. The female bud is bagged. When its stigma becomes receptive, the bag is removed, and suitable pollen is dusted on it.
- When pollen falls on the stigma, the pollen tube which forms enters one of the synergids and issues two male gametes.
- One of the male gametes fuses with the egg cell in a process called syngamy and forms a zygote.
- The other male gamete fuses with the polar nuclei to form a primary endosperm nucleus (PEN), which is triploid. This process is called triple fusion.
- The entire process is called double fertilization because two types of fusion take place—syngamy and triple fusion. This is characteristic of flowering plants.
- Once triple fusion is complete, the central cell transforms into the primary endosperm cell (PEC).
- The primary endosperm nucleus develops into the endosperm, whereas the zygote gives rise to the embryo.
These events comprise the development of the embryo and the endosperm, and the maturing of ovules into seeds and ovaries into fruits.
Formation of the Endosperm
- The cells of the endosperm give nutrition to the developing embryo, so the endosperm forms before the embryo is formed.
- During free nuclear endosperm, the primary endosperm nucleus divides several times to form free nuclei.
- A cellular endosperm is formed after the development of cell walls.
- In peas and beans, the endosperm is entirely consumed by the developing embryo. In coconut and castor, the endosperm remains in the mature seed.
Development of the Embryo
- The zygote is located at the micropylar end of the embryo sac. This is where embryo development takes place.
- First, the pro-embryo is formed and then the mature, globular, heart-shaped embryo is formed.
- A typical dicot embryo contains an embryonal axis and two cotyledons.
- The part of the embryonal axis above the level of the cotyledons is called epicotyl and contains the plumule or shoot tip. The part of the embryonal axis below the level of the cotyledons is called the hypocotyl and contains the radicle or root tip. The root tip is protected by the root cap.
- A monocot embryo has only one cotyledon. In grass, this single cotyledon is called scutellum and is located to one side of the embryonal axis. Towards the lower end of the embryonal axis lies the radicle and the root cap, which is covered by the coleorrhiza.
- The epicotyl lies above the scutellum’s level and the shoot apex and leaf primordia are enveloped in hollow structures called coleoptiles.
Seeds and Fruits
Development of Seeds
- In angiosperms, the final stage of sexual reproduction is the development of seeds.
- When fertilized ovules develop inside the fruit, they form seeds.
- A seed has the following parts—seed coat, cotyledons, and embryonal axis.
- Seeds may be albuminous (presence of endosperm e.g. wheat, maize) or non-albuminous (absence of endosperm since the growing embryo entirely consumes it e.g. beans and peas).
- Seeds of some plants like wheat and black pepper contain remnants of the nucellus known as perisperm.
- The seed coat is formed by the hardening of the integuments of the ovule. Water and oxygen enter the seed through the micropyle.
- The seed may become dormant if moisture is lost or it may germinate if favorable conditions exist.
Development of Fruits
- The ovary develops into a fruit.
- The walls of the ovary become the walls of the fruit (pericarp).
- Fruits may be fleshy (orange, mango) or may be dry (mustard, groundnut).
- In plants like strawberry and apple, floral parts other than the ovary (thalamus) participate in fruit formation. Such fruits are called false fruits. Fruits that form from the ovary are called true fruits.
- In plants like banana, fruits develop without fertilization and are called parthenocarpic
Apomixis and Polyembryony
- The process of formation of seeds without fertilization is known as apomixis.
- Apomixis mimics sexual reproduction but is actually a form of asexual reproduction.
- In some species, the diploid egg cell is formed without meiosis, which then develops into an embryo without fertilization. This is how apomixis occurs.
- In some varieties of mango and citrus, the cells of the nucellus divide and then jut into the embryo sac, eventually developing into embryos. Each ovule may thus have several embryos and this phenomenon is called polyembryony.
- Apomixis helps increase crop yield by many folds and helps develop hybrid varieties of vegetables and fruits.
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