5.7 THE SEED

5.7 THE SEED 

The ovules after fertilisation, develop into seeds. 

A seed is made up of a seed coat and an embryo. 

The seed coat, the outermost covering of a seed, has two layers, the outer testa and the inner tegmen. 

The hilum is a scar on the seed coat through which the developing seeds were attached to the fruit. 

Above the hilum is a small pore called the micropyle. It provides entry to the pollen tube into the ovule during fertilization.

The embryo is made up of an embryonal axis with a radicle (future root) at lower end & a plumule (future shoot) at other end and one (as in wheat, maize) or two cotyledons (as in gram and pea).

In some seeds such as castor, the endosperm formed as a result of double fertilisation, is a food storing tissue and called endospermic seeds. In plants such as bean, gram and pea, the endosperm is not present in mature seeds and such seeds are called non-endospermous. 

The cotyledon is the embryonic leaf (not true leaves which germinates from plumule) of a seed that becomes the first leaves of the embryonic plant while the endosperm is the storage of the nutrients needed by the embryo to develop into a seedling. The embryo get its nourishment from the endosperm while the young seedling gets its nourishment from the cotyledon.

5.7.1 Structure of a Dicotyledonous Seed 

The embryo, apart from an embryonal axis consists of two cotyledons. The cotyledons are often fleshy and full of reserve food materials. (Figure 5.14). Eg beans.

These seeds are non-endospermatic.

5.7.2 Structure of Monocotyledonous Seed

The embryo is  small and situated in a groove at one end of the endosperm. It consists of one large and shield shaped cotyledon known as scutellum and a short axis with a plumule and a radicle. 

The plumule and radicle are enclosed in sheaths which are called coleoptile (koleon - sheath + ptilon - feather/ shoot) and coleorhiza (koleon - sheath + rhizon - root) respectively (Figure 5.15).

In the seeds of cereals such as maize, the seed coat is membranous and generally fused with the fruit wall. 

Generally, these seeds are endospermic but some as in orchids are non-endospermic. 

The endosperm is bulky and stores food. The outer covering of endosperm separates the embryo by a proteinous layer called aleurone layer.