Germination, << `jur` muh NAY shuhn, >> is the sprouting of a seed. All seeds need moisture, oxygen, and warmth to germinate. These requirements, especially the need for warmth, vary from species to species.
The seeds of many kinds of tropical plants germinate within a few days if the proper conditions are present. Among most other plants, the seeds go through a period of dormancy (inactivity). Dormant seeds do not germinate for weeks or months, even if the conditions for sprouting seem ideal. Some dormant seeds must have their outer layer broken before they can germinate. Others require a period of cold temperatures or of additional daylight. Dormancy delays germination until conditions become favorable for growth. For example, it prevents germination during the fall, thus protecting the plants from being killed during winter.
Most seed-producing plants are flowering plants. Each seed of a flowering plant is covered by a dense protective coat. A layer of tissue called the endosperm lies inside the seed coat. The endosperm stores the food necessary for germination. The seed also includes the embryo, which develops into the mature plant. The embryo has a long structure called the axis, to which one or two seed leaves, known as cotyledons, are attached. The portion of the axis above the cotyledons, the epicotyl, consists of a young shoot with undeveloped leaves and stem. The portion below the cotyledons is called the hypocotyl. The lower end of the hypocotyl, called the radicle, develops into the plant’s first root.
Seeds become dehydrated during dormancy. When germination begins, they absorb great amounts of water from the soil. The water triggers chemical changes that enable the embryo to convert the stored food into energy needed for growth. The absorption of water also causes the embryo to swell and split open the seed coat. The radicle emerges and grows down into the soil, forming the first root. In some species of plants, the upper portion of the hypocotyl lengthens. It arches upward and pulls the cotyledons above the ground. In other plants, the cotyledons remain underground.
Among both groups of plants, the epicotyl grows upward above the ground. It produces stem growth and the plant’s first fully developed leaves. Germination is now complete because the young plant can gather water and minerals through its root and make food in its leaves.
