Sara Mendoza, age 14, of Galveston, Tex., for her question:
HOW DOES A SEED DEVELOP?
A plant's seeds are its most important part. Roots, leaves and flowers all exist so there can be seeds. Although some plants, such as ferns and mosses, do not have seeds and reproduce their kind by means of spores, most plants depend on seeds alone to reproduce.
The part of a flower that holds the tiny seed eggs is called the ovary. To develop into healthy seeds, the eggs must receive pollen from the same flower or the same kind of flower. Many changes then take place, and continue while the seed is developing.
A seed has three important parts: a protective outer skin, or seed coat; an embryo, which will become the new plant: and a food supply, or endosperm, usually in the form of one, two or many cotyledons, or seed leaves.
The cotyledons are stored with plant food. They are usually albumen and starch or oily matter. These nourish the embryo as it develops.
The seed coats of some seeds, such as the bean, have two structures: the hilum and the micropyle. The hilum is a small scar where the seed has attached to the seed stalk. The micropyle is a tiny home where the pollen tube that fertilizes the seed entered.
The epicotyl and the hypocotyl are tiny parts of the cotyledon. The spicotyl becomes the stem of the young plant. The hypocotyle develops into the plant's first root.
When a bean is planted, it swells with moisture so that it bursts from its seed coat. Then the embryo starts to grow. The root tip pushes through the eye of the bean. The embryo stem does not lengthen until it has rootlets to anchor it firmly in the earth.
The bean then arches up through the crust of the earth andbrings up the delicate plant bud protected by the tough cotyledons. As the stem strengthens, the cotyledons open wide and the bud is exposed to the light and air it needs for growth.
To germinate, or sprout, a seed must have moisture, air and the right temperature. Some need a lot of moisture while others need only a little.
Some seeds will sprout in the cold ground of early spring while others must have sun warmed ground that becomes just right in late spring or early summer.
Seeds come in all sizes, shapes and colors. They range from tiny poppy seeds, with many thousand found in one small pod, to the single seed of the coconut tree that can weigh up to 22 pounds.
The size of the seed has no bearing on the size of the plant that will come from it. The giant California redwood tree, for example, comes from a very small seed.