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Sexual reproduction is a kind of reproduction, in which two parents (one male other female) are involved in the formation of new individuals.
These parents produce gametes (specialized haploid cells) which on fertilization give rise to new individual. The individual is called hybrid.
For sexual reproduction, angiosperms possess reproductive organs. These organs are the flowers. To understand reproduction in plants, student must read the following basic concepts very carefully.
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Basic concept of sexual reproduction in angiosperms:
(a) In all categories of higher plants, two distinct phases are found in the life cycle.
These are:
(i) Sporophyte phase (2n) and
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(ii) Gametophyte phase (n)
(b) Both these phases regularly alternate with each other.
(c) In lower plants like Bryophytes, gametophyte phase is a dominant phase. But in higher plantslikepteridophyt.es, gymnosperms and angiosperms, the dominant and chief phase is sporophyte phase.
(d) In angiosperms gametophyte phase is highly reduce and microscopic.
1. Sporophyte phase:
(a) It is the diploid phase (always)
(b) It is developed from diploid zygote. Therefore zygote represents the first cell sporophyte phase.
(c) Sporophytic phase is called so, because it gives rise to spores.
(d) Spores are haploid cells (but these not the gametes, so students should note this fact)
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(e) A sporophyte (since it is diploid) produces haploid spores with the help of meiosis (also called reductional division)
(f) In higher plants, like angiosperms, spores are of two types. Male plants produce small sized spores, called microspores (Or pollen grains). Whereas the female plant (part) produces larger spores, called megaspores (microspores). (So student should note that pollen grains or microspores are not the male gametes or sperms).
(g) In angiosperms, the entire standing plant body is sporophytic (diploid).
(h) Gametophytic tissues are highly reduced and develop inside the small part of flower.
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(i) Microspores are developed inside microsporangia (anthers), while megaspores develop inside megasporangium (ovule), within ovaries.
2. Gametophytic phase:
(a) It is the haploid phase of the plant.
(b) It is very reduced phase in higher plants like angiosperms, but is very important.
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(c) It develops from spores (which are haploid structures). Therefore spores represent the first cells of gametophytic phase.
(d) Since there are two types of spores – microspores (pollen grains or male spores) and megaspore (or female spores), the gameteophytes are also of two types – Male gametophyte (it develops from microspore) and female gametophyte (it develops from megaspore).
(e) Gametophytes on development give rise to gametes.
(f) Male gametophyte produces male gametes (sperms), while female gametophyte produces female gametes (egg cell or ovum).
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(g) In angiosperms, the mature male gametophyte consists only of 3 cells. (It contains one tube cells and 2 sperms).
(h) Its development starts when the microspores are within the anther but gets completed only when it is transferred to the stigma of the flower (through pollination).
(i) The mature female gametophyte in angiosperms is mostly 7 celled (but having 8 nuclei),
(j) It is also called as embryo sac. It contains, an egg cell, 2 synergic cells, 3 antipodal cells and one large secondary nuclei.
The gametes from two types of gametophytes unite (fertilize) to produce a diploid cell, called zygote, which sets seeds and again start the sporophyte phase.
In this way a regular alternation between sporophyte and gametophyte stage is clearly apparent in angiosperms.
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Reproductive Organs:
Flower is the reproductive organ of angiosperrnic plants. It is found only in angiosperms. It contains male and female reproductive organs, which are responsible for the production of gametes (sex cells). In general, a flower consists of four whorls, calyx, corolla, androecium and gynoecium. Calyx and corolla are called accessory whorls, while other two are reproductive whorls. Androecium is called male whorl, as it contains stamens, male sex organs. Gynoecium is called female whorl as it contains pistils (or carpels), the female sex organs.
