A seed is a small embryonic plant enclosed inside a seed coat. It is the ripened and fertilised ovule of gymnospermic and angiospermic plants. A seed has a three parts:
The embryo is an undeveloped plant inside a seed from which a new plant develops. All seeds do not have the same size, shape and colour. We will be looking at the embryo of a pea, gram and bean seed to identify their different parts.
Plant embryos in seeds have structures called cotyledons. A cotyledon is the central portion of a seed embryo to which the epicotyls- the immature shoot, and the radicle- the immature roots, are attached.
Plants are classified according to the number of cotyledons present in the embryo. If the embryo has one cotyledon (monocotyledon) it is a monocot plant, and if there are two cotyledons (dicotyledon), it is a dicot plant.
Based on the type and location of storage materials, seeds can be subdivided into endosperms and non endosperms.
Endospermic seeds are those that have an endosperm in the mature seed. It is fleshy, oily, surrounds the embryo, and functions as the sole food storage organ. Inside the seed coat, a thin and papery cotyledon is present. Monocot plants have endospermic seeds.
Non-endospermic seeds do not have an endosperm in the mature seed. The cotyledons are thick and fleshy, and function as the sole food storage organs. Dicot plants have non-endospermic seeds.
Germination is the process in which the seed breaks apart, and the embryo inside begins to grow with the help of water and nutrients from the soil. Seeds normally begin to germinate when they get an appropriate range of soil, temperature and when water and oxygen are available. During germination the seeds absorb water either through the micropyle (pore like opening), if it is present, or through the testa (outer seed coat) when it is permeable. The entry of water activates the embryo cells.
The pea, gram and bean seeds have two seed coats that envelope and protect the embryo. The outer seed coat called the testa is thick and strong. The inner coat is called tegmen, which is thin and membranous.
The outer seed coat has a notch on one side and a scar along the notch, called the hilum. It is the hilum that attaches the seed to the inner margin or the seed stalk. There is a small pore called micropyle located at one end of the hilum. The seed imbibes water through the microphyle.
Once soaked for the required time, the part of the plant that first emerges from the seed is the embryonic root known as the radicle. The radicle allows the seedling to anchor itself to the ground and start absorbing water.
After the root starts absorbing water, an embryonic shoot emerges from the seed. This shoot comprises of three main parts- the cotyledons (seed leaves), the hypocotyl (section of shoot below the cotyledons), and the epicotyl (section of shoot above the cotyledons).