BIOLOGY [5 Mark Question Answer]

Pollen grains deposited on the stigma absorb water and swell, due to which its exine ruptures and intine grows out as pollen tube. The pollen tube grows through the style by secreting enzymes that digest reserve food in the tissues of stigma and style. The vegetative nucleus moves to the tip of the pollen tube followed by the generative nucleus which divides into two male nuclei. Now the pollen tube enters the ovule through the micropyle and discharges the two male gametes into the embryo sac.

Pollination in Salvia - Insects bring about pollination in Salvia. Flowers of Salvia have corolla whose upper part is blipped and Iower part is tubular where two stamens are attached. Upper lip forms a hood like structure while lower lip serves as a landing platform for insects. Of the two stamens, the fertile one is situated under the hood like upper lip of corolla. When an insect sits on the lower lip and pushes its proboscis into the corolla tube for collecting nectar, its back touches the fertile anther, which gets dusted with pollen grains. When this insect visits another flower for nectar, the style bends down and touches the back of the insect, as a result pollination occurs.

Pollination in Maize - Pollination in maize is brought about by the wind. The maize plant is monoecious. The cluster of male flowers called tassel are borne terminally whereas the female inflorescence i.e. cobs are borne laterally at lower levels. Silky thread-like styles project out of the female flowers. Male flowers produce many heavy pollen grains, which get released during mild winds and fall down vertically. They fall on the stigma, bringing about pollination.

Adaptations required by cross-pollinated plants are:
(i) Unisexuality - The stamens and carpels are found in different flowers. The male and female flowers may be borne on the same or different plants.
(ii) Dichogamy - In bisexual flowers, stamens and carpels mature at different times. It is of two kinds:
(a) Protandry wherein stamens mature before carpels. E.g - jasmine
(b) Protogyny wherein carpels mature before stamens. E.g. - Rose
(iii) Heterostyly - Here the style is either longer or shorter, thereby preventing self-pollination.
(iv) Herkogamy - Stigma, and stamen mature at the same time, but some type of barrier prevents self-pollination. E.g. - In caryophyllaceous flower, the stigma projects beyond the stamens so that pollens cannot fall on it.
(v) Self-sterility - Pollen of one flower cannot fertilize the female gametes of the same flower.