A Civic Biology, Presented in Problems. George W. Hunter
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.
Test for Starch.—If we boil water with a piece of laundry starch in a test tube, then cool it and add to the mixture two or three drops of iodine solution,[3] we find that the mixture in the test tube turns purple or deep blue. It has been discovered by experiment that starch, and no other known substance, will be turned purple or dark blue by iodine. Therefore, iodine solution has come to be used as a test for the presence of starch.
Test for Starch.
Starch in the Bean.—If we mash up a little piece of a bean cotyledon which has been previously soaked in water, and test for starch with iodine solution, the characteristic blue-black color appears, showing the presence of the starch. If a little of the stained material is mounted in water on a glass slide under the compound microscope, you will find that the starch is in the form of little ovoid bodies called starch grains. The starch grains and other food products are made use of by the growing plant.
Test for protein.
Test for Oils.—If the substance believed to contain oil is rubbed on brown paper or is placed on paper and then heated in an oven, the presence of oil will be known by a translucent spot on the paper.
Protein in the Bean.—Another nutrient present in the bean cotyledon is protein. Several tests are used to detect the presence of this nutrient. The following is one of the best known:—
Place in a test tube the substance to be tested; for example, a bit of hard-boiled egg. Pour over it a little strong (60 per cent) nitric acid and heat gently. Note the color that appears—a lemon yellow. If the egg is washed in water and a little ammonium hydrate added, the color changes to a deep orange, showing that a protein is present.
If the protein is in a liquid state, its presence may be proved by heating, for when it coagulates or thickens, as does the white of an egg when boiled, protein in the form of an albumin is present.
Another characteristic protein test easily made at home is burning the substance. If it burns with the odor of burning feathers or leather, then protein forms part of its composition.[4]
A test of the cotyledon of a bean for protein food with nitric acid and ammonium hydrate shows us the presence of this food. Beans are found by actual test to contain about 23 per cent of protein, 59 per cent of carbohydrates, and about 2 per cent oils. The young plant within a pea or bean is thus shown to be well supplied with nourishment until it is able to take care of itself. In this respect it is somewhat like a young animal within the egg, a bird or fish, for example.
Beans and Peas as Food for Man.—So much food is stored in legumes (as beans and peas) that man has come to consider them a very valuable and cheap source of food. Study carefully the following table:—
| Nutrients Furnished for Ten Cents in Beans and Peas at Certain Prices per Pound | |||||
|---|---|---|---|---|---|
| Ten Cents will pay for— | |||||
| Food Materials as Purchased | Prices per Pound | Total Food Material | Protein[TN1] | Fat | Carbo- hydrates |
| Cents | Pounds | Pounds | Pounds | Pounds | |
| Kidney beans, dried | 5 | 2.00 | 0.45 | 0.04 | 1.19 |
| Lima beans, fresh, shelled | 8 | 1.25 | .04 | — | .12 |
| Lima beans, dried | 6 | 1.67 | .30 | .03 | 1.10 |
| String beans, fresh, 30 cents per peck |
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