Seeds: Structure and Germination

A seed is a fertilised ovule that has ripened. It has three essential parts. The seed coat (an outer tough testa and an inner thin tegmen) protects the contents. On the coat you can see the hilum, a scar where the seed was joined to the fruit, and the micropyle, a tiny pore beside it. Inside lies the embryo, made of the plumule (which grows into the shoot), the radicle (which grows into the root), and one or two cotyledons. Food for the early growth is stored either in the cotyledons or in a separate tissue called the endosperm.

Seeds are grouped by the number of cotyledons. A bean or pea seed has two cotyledons and is a dicot seed; its thick cotyledons store the food, so no endosperm is left. A maize grain has a single cotyledon and is a monocot seed; here the food is stored in the endosperm, and the single shield-shaped cotyledon, called the scutellum, absorbs that food and passes it to the embryo. In maize the plumule is wrapped in a protective sheath, the coleoptile, and the radicle in a sheath called the coleorhiza.

Germination is the process by which the embryo in a resting seed becomes active and grows into a seedling. Three conditions are essential. Water softens the seed coat, lets the seed swell, and activates the enzymes that digest the stored food. Oxygen is needed for respiration, which releases the energy the embryo uses to grow; this is why seeds buried too deep fail, as they cannot get enough air. An optimum temperature (neither too cold nor too hot) is needed because the enzymes only work in a suitable range. Light is not needed by most seeds to start, though some seeds do require it and a few are stopped by it.

Once a seed germinates, the cotyledons may or may not come above the ground. In epigeal germination, the hypocotyl (the part of the stem below the cotyledons) grows fast and arches into a loop that pushes the cotyledons up above the soil, where they often turn green; the bean and castor show this. In hypogeal germination, the epicotyl (the part above the cotyledons) grows instead, so the cotyledons stay below the soil and only the plumule is pushed up; the pea, gram and maize show this. The difference simply depends on which region of the young stem elongates.