A Civic Biology, Presented in Problems

by George W. Hunter

Three views of a kidney bean, the lower one having one cotyledon removed to show the hypocotyl and plumule.

If we open a bean pod, we find the seeds lying along one edge of the pod, each attached by a little stalk to the inner wall of the ovary. If we pull a single bean from its attachment, we find that the stalk leaves a scar on the coat of the bean; this scar is called the hilum. The tiny hole near the hilum is called the micropyle. Turn back to the figure (page [54]) showing the ovule in the ovary. Find there the little hole through which the pollen tube reached the embryo sac. This hole is identical with the micropyle in the seed. The thick outer coat (the testa) is easily removed from a soaked bean, the delicate coat under it easily escaping notice. The seed separates into two parts; these are called the cotyledons. If you pull apart the cotyledons very carefully, you find certain other structures between them. The rodlike part is called the hypocotyl (meaning under the cotyledons). This will later form the root (and part of the stem) of the young bean plant. The first true leaves, very tiny structures, are folded together between the cotyledons. That part of the plant above the cotyledons is known as the plumule or epicotyl (meaning above the cotyledons). All the parts of the seed within the seed coats together form the embryo or young plant. A bean seed contains, then, a tiny plant protected by a tough coat.

Food in the Cotyledons.—The problem now before us is to find out how the embryo of the bean is adapted to grow into an adult plant. Up to this stage of its existence it has had the advantage of food and protection from the parent plant. Now it must begin the battle of life alone. We shall find in all our work with plants and animals that the problem of food supply is always the most important problem to be solved by the growing organism. Let us see if the embryo is able to get a start in life (which many animals get in the egg) from food provided for it within its own body.

Organic Nutrients.—Organic foods (those which come from living sources) are made up of two kinds of substances, the nutrients or food substances and wastes or refuse. An egg, for example, contains the white and the yolk, composed of nutrients, and the shell, which is waste. The organic nutrients are classed in three groups.

Starch grains in the cells of a potato tuber.

Carbohydrates, foods which contain carbon, hydrogen, and oxygen in a certain fixed proportion (C₆H₁₀O₅ is an example). They are the simplest of these very complex chemical compounds we call organic nutrients. Starch and sugar are common examples of carbohydrates.

Fats and Oils.—These foods are also composed of carbon, hydrogen, and oxygen in a proportion which enables them to unite readily with oxygen.

Proteins.—A third group of organic foods, proteins, are the most complex of all in their composition, and have, besides carbon, oxygen, and hydrogen, the element nitrogen and minute quantities of other elements.