Seed and Fruit Formation (10th-Biology-Lesson-16.4)
Seed and Fruit Formation, Ovule after fertilization forms seeds. Zygote forms the embryo by mitosis. This consists of tiny radicle, a plumule with one or two short special leaves called cotyledons. Cotyledons store the food of endosperm in some seeds and become thick e.g. bean seed. The outer integument of ovule forms the testa and the inner integument forms tegment. Testa and tegmen constitute seed coat. Seeds provide protection and nourishment to the embryo.
After the formation of seeds the ovary wall becomes fleshy or scaly and forms fruit wall. The floral parts i.e.sepals, petals, stamen, stigma shrivel and fall off. This ripened ovary with seeds is the fruit.
Parthenocarpic fruits may be formed by the ovary in which no fertilization occurs as it happens in fruits of banana, seedless varieties of grapes etc. in some other plants, unfertilized egg develops into new plant and this process is called parthenogenesis.
16.4.1 Dispersal of seeds and fruits
The seeds are the embryos of flowering plants in dormant stage. Fruits protect the seeds and help in the dispersal of the plant i.e. seeds and fruits may be carried a long distance from the parent plant. This reduces competition for light, water and other nutrients among the members of same species. The seeds and fruits may be dispersed by wind, by animals or by water.
1. Dispersal by wind
The smaller seeds and fruits are carried by wind to distant places. These seeds and fruits are light in weight and may also have wings or hairs. This helps them to be carried away by wind, away from the parent plant.
Examples are seeds of cotton, Bombax (sumbal), calotropis, grasses and Dandelion; and fruits of maple etc.
2. Dispersal by animals
The fruits and seeds of some plants have hooks or spines, which may be entangled in the fur of many mammals and are taken away from the site of parent plant. For example, famedicago (maina), cocklebur, and Achyranthus (Puthkanda)
Some fruits and seeds are eaten by animals. The seeds are not digested and are passed out of the body of the animal alongwith faeces. Thus these are carried away from the site of the parent plant to new growing places e.g. guava, mulberry and ficus.
3. Dispersal by water
The fruits and seeds of some plants which grow in or near water are dispersed by water. The fruits of these plants may contain spongy tissues or fibrous fruit walls which allow them to float on the surface of water to be carried to distant places e.g fruit of lotus, coconut, and daffodils.
A (i) Seeds of dandelion have filamentous tufts that are dispersed by wind.
A (ii) Winged fruits of maple dispersed by wind.
Fig.16.10 Examples of fruits and seeds. (a) dispersed by wind (b) dispersed by animals (c) dispersed by water