Sexual Reproduction in Flowering Plants: common doubts, answered
The questions students ask most often about Sexual Reproduction in Flowering Plants, each with a short answer. For the full chapter, read the Sexual Reproduction in Flowering Plants notes.
The flower and its parts
Read this section in the notes →What is the difference between a stamen and a pistil?
The stamen is the male whorl and the pistil is the female whorl of a flower. A stamen has an anther, which makes pollen, held on a filament. A pistil has a stigma that receives pollen, a style that carries it, and an ovary below that holds the ovules which later become seeds.
Stamen, microsporangium and microsporogenesis
Read this section in the notes →Why is a typical anther called bilobed, dithecous and tetrasporangiate?
Bilobed means it has two lobes, dithecous means each lobe has two chambers (thecae), and tetrasporangiate means there are four pollen sacs in all. Each lobe carries two microsporangia, which is why the total is four. These sacs are where pollen mother cells divide to form pollen grains.
What is the function of the tapetum in an anther?
The tapetum feeds the developing pollen grains. It is the innermost wall layer of a microsporangium, lying right next to the sporogenous cells, and its cells are often multinucleate, which suits their heavy secretory job. The outer layers, namely epidermis, endothecium and middle layers, are the ones that protect the anther and help it open.
How is microsporogenesis different from megasporogenesis?
Microsporogenesis makes four functional microspores from each pollen mother cell, whereas megasporogenesis makes four megaspores of which only one survives. Both begin with meiosis of a diploid mother cell. Pollen therefore forms in large numbers inside anthers, while each ovule normally produces a single functional megaspore that goes on to form the embryo sac.
The pollen grain
Read this section in the notes →Why does the pollen grain wall resist heat, acid and enzymes?
The outer wall, called exine, is made of sporopollenin, one of the most resistant organic materials known. It withstands high temperature and strong acids and alkalis, and no enzyme known to biologists breaks it down. This is why pollen survives as fossils. The only places where exine is missing are the germ pores, through which the pollen tube later emerges.
What is the difference between the vegetative cell and the generative cell of a pollen grain?
The vegetative cell is large, rich in food and has an irregularly shaped nucleus, and it forms the pollen tube. The generative cell is small, floats in the vegetative cytoplasm and divides to produce two male gametes. So the vegetative cell is the supporter and the generative cell is the source of the sperm nuclei.
Pistil, ovule and megasporogenesis
Read this section in the notes →What are hilum, micropyle and chalaza in an ovule?
The hilum is the point where the ovule joins its stalk, the funicle. The micropyle is the small opening at the tip, through which the pollen tube usually enters. The chalaza is the base of the ovule, opposite the micropyle. The nucellus sits inside, covered by integuments, and holds the embryo sac.
The embryo sac
Read this section in the notes →Why is the embryo sac called 7-celled but 8-nucleate?
It has eight nuclei but only seven cells because two polar nuclei lie together in one large central cell instead of being separated by walls. The other six nuclei each form a cell: three antipodals at the chalazal end, and the egg with two synergids at the micropylar end. Hence seven cells containing eight nuclei.
What is the egg apparatus and why do synergids have a filiform apparatus?
The egg apparatus is the group of one egg cell and two synergids lying at the micropylar end of the embryo sac. The synergids carry thickened, finger-like cell-wall projections called the filiform apparatus, which guides the pollen tube into the synergid and so towards the egg. It helps ensure the male gametes reach the right cells.
How does one megaspore become an 8-nucleate embryo sac?
The functional megaspore nucleus divides mitotically three times without wall formation, giving 2, then 4, then 8 free nuclei. Walls then form, organising six of the nuclei into cells at the two ends, while the two polar nuclei stay in the large central cell. Because the whole embryo sac comes from a single megaspore, this pattern is called monosporic development, the commonest type in flowering plants.
Kinds of pollination and its agents
Read this section in the notes →What is the difference between autogamy, geitonogamy and xenogamy?
Autogamy is transfer of pollen to the stigma of the same flower. Geitonogamy is transfer to another flower on the same plant. Xenogamy is transfer to a flower on a different plant. Only xenogamy brings genetically different pollen, so only it adds new combinations of genes to the offspring.
Why is geitonogamy genetically autogamy but ecologically cross-pollination?
The pollen moves between two different flowers, usually carried by an agent such as an insect or wind, so ecologically it behaves like cross-pollination. But both flowers belong to the same plant, so the pollen and the stigma are genetically identical, which makes the result as inbred as autogamy. That is the reason for the dual description.
What are cleistogamous flowers and why do they always set seeds?
Cleistogamous flowers never open, so the anthers and the stigma stay enclosed together and pollination occurs inside the closed bud. Viola, Oxalis and Commelina produce them. Pollination does not depend on any outside agent, hence they guarantee seed set even when pollinators are absent, though the offspring show little variation.
Are water lily and water hyacinth pollinated by water?
No, they are not. Their flowers rise above the water surface and are pollinated by insects or wind, like land plants. Water pollination is rare, found in about 30 genera and mostly in monocots, with Vallisneria, Hydrilla and Zostera as the NCERT examples. Living in water does not make a plant water-pollinated.
Outbreeding devices and pollen-pistil interaction
Read this section in the notes →How do monoecious and dioecious plants differ in preventing self-pollination?
Monoecious plants such as maize and castor bear separate male and female flowers on one plant, which prevents autogamy but not geitonogamy. Dioecious plants such as papaya keep male and female flowers on different plants, which prevents both. So dioecy is the more complete outbreeding device.
Double fertilisation
Read this section in the notes →What is double fertilisation and why is it called that?
Double fertilisation is two fusion events in the same embryo sac, which is why it has that name. One male gamete fuses with the egg (syngamy) to form a diploid zygote. The other fuses with the two polar nuclei (triple fusion) to form a triploid primary endosperm nucleus. It is a hallmark of flowering plants.
Endosperm and embryo
Read this section in the notes →Why is endosperm triploid but the embryo diploid?
The embryo comes from syngamy, a fusion of one haploid male gamete with one haploid egg, giving 2n. The endosperm comes from triple fusion, where one male gamete joins two haploid polar nuclei, giving 3n. The endosperm then serves as stored food nourishing the growing embryo, and it develops before the zygote starts dividing.
Is coconut water endosperm?
Yes. The tender coconut water is free-nuclear endosperm, where the primary endosperm nucleus has divided many times without forming cell walls, so the nuclei float in the liquid. The white kernel you eat is cellular endosperm, formed later when walls develop around the nuclei. Both are therefore the same tissue at different stages.
Seed and fruit
Read this section in the notes →What is the difference between albuminous and non-albuminous seeds?
Albuminous seeds keep a leftover endosperm at maturity, as in wheat, maize, barley and castor. Non-albuminous seeds, such as pea and groundnut, have no residual endosperm because the developing embryo used it all up. In those seeds the stored food ends up in the cotyledons instead.
What is the difference between perisperm and pericarp?
Perisperm is the persistent remnant of nucellus inside a seed, found in black pepper and beet. Pericarp is the wall of a fruit, formed from the ovary wall. One lives inside the seed and the other surrounds the whole fruit, so they come from different parts of the ovule and ovary.
What are false fruits and parthenocarpic fruits?
A false fruit is one in which a part other than the ovary, usually the thalamus, forms most of the fruit, as in apple, strawberry and cashew. A parthenocarpic fruit develops without fertilisation and has no seeds, as in banana. The first is defined by its origin and the second by its seedlessness.
Apomixis and polyembryony
Read this section in the notes →What is apomixis and why is it useful in agriculture?
Apomixis is seed formation without fertilisation, so the seeds are clones of the mother plant. It is useful because a hybrid with desirable traits can be multiplied for seed without segregation of characters in later generations. Farmers could then keep growing hybrid seed every year instead of buying fresh seed. Some grasses and Asteraceae members do it naturally.
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