Ans. Development and Structure of Seed
The evolution of seed has been proved as an important step in the success and spread of flowering plants, as compared to the seedless plants like mosses and : ferns.
Development of Seed
After fertilization in the female gametophyte, zygote divides repeatedly by mitosis and develops into an embryo. At this stage (in gymnosperms and anglosperms), ovule changes into seed. The formation of seed completes the process of sexual reproduction in seed plants.
Structure of Seed
Angiosperm seeds consist of three distinct parts:
The embryo formed from zygote,
The endosperm tissue formed from endosperm nucleus, and
iii)The seed coat which develops from the wall of ovule (integument).
1) Seed Coat
Seed coat (or testa ) develops from the integument, originally surrounding the ovule. It may be a paper-thin layer (e.g. peanut) or thick and hard (e.g. coconut). Seed coat protects embryo from mechanical injury and from drying out. There is a scar on seed coat, called hilum. It is where the seed is attached to ovary wall (fruit). At one end of hilum, there is micropyle. This is the same opening through which the pollen tube entered ovule. Seed uses it for the absorption of water.
Embryo is actually an immature plant. It consists of a radical, a plumule and one or two-cotyledons (seed leaves). The radical of embryo develops into new root while the plumule develop into new shoot.The embryonic stem below the point of attachment of cotyledon/(s) is called epicotyl. The embryonic stem below the point of attachment is hypocotyls.
Within seed, there is a store of nutrients for the seedling that will grow from embryo. In angiosperms, the stored food is derived from the endosperm tissue. This tissue is rich in,oil or starch and protein. In many seeds, the food of the endosperm is absorbed and stored by cotyledons. The form of the stored nutrients in seeds varies depending on the kind of plant. Figure 14.15