CBSE Class 10 Science 2017 · Paper 31/2/1 · Question 20

Unisexual flower: papaya (or watermelon). Bisexual flower: Hibiscus (or mustard). Self-pollination is the transfer of pollen from the anther to the stigma of the same flower; it usually needs no outside agent, and since the germ-cells come from one plant the offspring are usually similar to the parent. Cross-pollination is the transfer of pollen from the anther of one flower to the stigma of another flower, usually on another plant of the same species; it needs an agent such as wind, water, insects or birds, and when the pollen comes from another plant it combines germ-cells of different plants and gives more variation. When a pollen grain lands on a suitable stigma it germinates: a pollen tube grows out of it and travels through the style to reach the ovule in the ovary, carrying the male germ-cell. The male germ-cell fuses with the female germ-cell (egg) in the ovule: this is fertilisation, and it forms a zygote. The zygote divides several times to form an embryo within the ovule; the ovule develops a tough coat and is gradually converted into a seed; the ovary grows rapidly and ripens to form a fruit; and the petals, sepals, stamens, style and stigma may shrivel and fall off.