Germination is the resumption of growth and development by a spore or plant embryo contained in a seed after a period of reduced metabolic activity or dormancy. Conditions required for germination include an adequate water supply, sufficient oxygen, and a favorable temperature. Rapid uptake of water followed by increased rate of respiration are often the first signs of germination. During germination, stored food reserves are rapidly used up to provide the energy and raw materials required for the new growth. The embryonic root and shoot which break through the seed coat are called the radicle and plumule, respectively. There are two general forms of germination: hypogeal and epigeal (see below). In the former, the seed leaves, or cotyledons, remain below the ground, as in the broad bean, while in the latter they are taken above the ground and become the first photosynthetic organs, as in the castor oil seed.

Epigeal germination

A castor-oil seed germinating in most soil. Water has been absorbed and the seed-coat has split; through the opening thus formed the radicle grows out and turns downward into the soil.

The radicle grows and branches and becomes the main root.

The hypocotyl lengthens and the plumule grows upward into the air, carrying the seed-coat with it. Meanwhile the cotyledons are absorbing the food-store and passing it to all parts of the plant. When the food-store is finished the cotyledons then start opening out and soon become the first leaves.

Between the two cotyledons the plumule appears and the cotyledons turn green and start functioning as leaves. The plant can now start to make its own food by photosynthesis with the aid of chlorophyll, and by absorbing salts from the soil.