Microspore mother cells in the anther undergo meiosis to form microspores that develop into pollen grains, the male gametophytes. The megaspore mother cell in the nucellus of the ovule undergoes meiosis, and the functional megaspore develops into the embryo sac, the female gametophyte.
The male gametophyte develops in the anther, and the female gametophyte develops in the ovule.
In microsporogenesis, each diploid pollen mother cell undergoes meiosis to form four haploid microspores. In megasporogenesis, the diploid megaspore mother cell undergoes meiosis to form four haploid megaspores. In most angiosperms, three megaspores degenerate and one functional megaspore forms the embryo sac.
Microsporogenesis forms microspores from microspore mother cells in the anther; megasporogenesis forms megaspores from the megaspore mother cell in the ovule. Both involve meiosis. Microsporogenesis produces a microspore tetrad, and megasporogenesis produces a megaspore tetrad, usually with one functional megaspore.
Sporogenous tissue gives rise to pollen mother cells. Each pollen mother cell undergoes meiosis to form a microspore tetrad. Microspores separate and develop into pollen grains. The generative cell in the pollen grain divides to form two male gametes.
Sporogenous tissue -> pollen mother cell -> microspore tetrad -> pollen grain -> male gametes.
The ovule is attached to the placenta by the funicle. The junction of the ovule and funicle is the hilum. One or two integuments surround the nucellus but leave a small opening called the micropyle. The basal region opposite the micropyle is the chalaza. The embryo sac lies within the nucellus. In an anatropous ovule, the funicle is fused with the body of the ovule to form the raphe.
A typical angiosperm ovule has a funicle, hilum, integuments, micropyle, nucellus, embryo sac, chalaza and often a raphe in anatropous ovules.
After meiosis in the megaspore mother cell, four megaspores are formed. In the common Polygonum type, three degenerate and only one functional megaspore develops into the embryo sac. Since only one megaspore contributes, it is called monosporic development.
Monosporic development is the development of the female gametophyte from a single functional megaspore.
The functional megaspore undergoes three mitotic divisions to form eight nuclei. Three nuclei at the chalazal end form antipodal cells. Three nuclei at the micropylar end form the egg apparatus, consisting of one egg cell and two synergids. The remaining two nuclei move to the centre as polar nuclei in one central cell. Thus there are seven cells but eight nuclei.
The mature embryo sac is 7-celled and 8-nucleate because it has three antipodal cells, two synergids, one egg cell and one central cell containing two polar nuclei.
Chasmogamous flowers permit pollen transfer between flowers. Cleistogamous flowers do not open, so their anthers and stigma remain enclosed within the same flower. This ensures self-pollination and prevents entry of pollen from another flower.
Chasmogamous flowers are open flowers with exposed anthers and stigma. Cross-pollination cannot normally occur in cleistogamous flowers because they remain closed.
In some species, pollen release and stigma receptivity do not occur at the same time. In others, anthers and stigma are placed so that self-pollen cannot reach the stigma. Self-incompatibility prevents germination or growth of self-pollen even if it reaches the stigma.
Two strategies are temporal separation of pollen release and stigma receptivity, and spatial separation of anther and stigma positions. Self-incompatibility is another common strategy.
The pistil can recognise pollen from the same plant or genetically similar plants. Such pollen is prevented from germinating on the stigma or from growing through the style. Without fertilisation, the ovule cannot form a seed.
Self-incompatibility is a genetic mechanism that prevents self-pollen from fertilising the ovule. In self-incompatible species, self-pollen is rejected, so pollen germination or pollen tube growth fails and fertilisation does not occur.
In plant breeding, anthers are removed from a bisexual flower before dehiscence. The flower is then bagged to prevent pollination by stray pollen. When the stigma becomes receptive, selected pollen is applied, and the flower is rebagged. This ensures that the desired cross is made.
Bagging is covering an emasculated flower with a bag to prevent contamination by unwanted pollen. It is useful in controlled hybridisation.
After the pollen tube releases two male gametes into the embryo sac, one gamete fuses with the egg to form the zygote. The other gamete moves to the central cell and fuses with the two polar nuclei or secondary nucleus. Since three haploid nuclei participate, the event is called triple fusion.
Triple fusion is the fusion of one male gamete with the two polar nuclei in the central cell of the embryo sac, forming the triploid primary endosperm nucleus.
After fertilisation, the primary endosperm nucleus divides to form endosperm. The zygote usually starts dividing only after some endosperm is formed. This ensures that the developing embryo has a nutritive tissue available.
The zygote remains dormant for some time so that endosperm can develop first and provide nourishment for embryo development.
The hypocotyl ends in the radicle and forms the embryonic root region, while the epicotyl ends in the plumule and forms the shoot region. In monocots, the coleoptile covers the shoot apex and the coleorrhiza covers the root apex. Integuments are present before fertilisation around the ovule and later harden into the testa. Perisperm is residual nucellus in some seeds, whereas pericarp encloses the fruit.
(a) Hypocotyl is the embryonic axis below the cotyledons; epicotyl is above the cotyledons. (b) Coleoptile protects the plumule; coleorrhiza protects the radicle. (c) Integuments are ovule coverings; testa is the seed coat formed from integuments. (d) Perisperm is persistent nucellus; pericarp is the fruit wall formed from the ovary wall.
A true fruit develops from the ovary after fertilisation. In apple, floral parts other than the ovary, especially the thalamus, become the major fleshy edible portion. Therefore it is called a false fruit or pseudocarp.
Apple is called a false fruit because the thalamus also contributes to fruit formation. In true fruits the ovary forms the fruit, while in apple the fleshy edible part develops mainly from the thalamus.
In controlled hybridisation, the selected female parent must receive only the desired pollen. If the flower is bisexual, its anthers are removed before pollen release. The flower is then bagged, pollinated with selected pollen when the stigma is receptive, and rebagged.
Emasculation is removal of anthers from a bisexual flower before they dehisce. A plant breeder uses it before artificial pollination to prevent self-pollination and ensure a desired cross.
Parthenocarpy produces fruits without fertilisation and therefore without seeds. Seedless fruits are easier to eat, have better consumer acceptance in many cases and can have commercial value. Hence fleshy edible fruits with many hard or inconvenient seeds are good targets.
Parthenocarpy would be selected for fruits where seedlessness is desirable, such as banana, grapes, watermelon, orange or tomato.
The tapetum is the innermost wall layer of the anther and surrounds the sporogenous tissue. It provides nutrition to developing pollen grains and contributes substances required for exine formation. Its secretions help form the tough outer pollen wall rich in sporopollenin.
Tapetum nourishes developing microspores and supplies materials, enzymes and precursors needed for pollen wall formation, including sporopollenin precursors.
In apomixis, embryos may develop without fusion of gametes. If apomixis is introduced into hybrid crops, farmers could reuse seeds without losing desirable hybrid characters, reducing the need to buy hybrid seed every season.
Apomixis is a form of asexual seed formation in which seeds are produced without fertilisation. It is important because it can preserve hybrid vigour and produce genetically identical seeds.