be adding weight to our body, and that added weight comes as the result of taking food into the body. What is true of a boy or girl is equally true of plants. If food is supplied in proper quantity and proportion, they will live and grow; if the food supply is cut off, or even greatly reduced, they will suffer and may die. From this, the definition which follows is evident. A food is a substance that forms the material for the growth or repair of the body of a plant or animal or that furnishes energy for it.

Nutrients. Organic food substances may be classed into a number of groups, each of which may be detected by means of its chemical composition. Such groups of food substances are known as nutrients. Let us now examine the nutrients.1

Carbohydrates. Starch and sugar are common examples of this group of substances. The former we find in our cereals, bread, cake, and most of our vegetables. Several forms of sugar are commonly used as food; for example, cane sugar, beet sugar, and glucose or grape sugar. Glucose, found as the natural sugar of grapes, honey, and fruits, is manufactured commercially by pouring sulphuric acid over starch. It is used as an adulterant for many kinds of foods, especially in sirups, honey, and candy.

Fats and Oils. Fats and oils form an important part of the composition of plants and animals. Examples of food in the form of fat are butter and cream, the oils in nuts, olives, and other fruits, and fat in animals.

Proteids. Nitrogenous foods, or proteids, contain the element nitrogen in addition to carbon, hydrogen, and oxygen of the carbohydrates and fats and oils. They include some of the most complex substances known to the chemist, and, as we shall see, have a chemical composition very near to that of living matter. Proteids occur in several different forms. White of egg, lean meat, beans, and peas are examples of substances composed in a large part of proteids.

Inorganic Foods. Water and various salts, some of which, as lime, may be found in drinking water, form important parts in the diet of plants and animals. Later we shall see that green plants, although they use precisely the same foods as we do, take into their bodies the chemical elements which form foods.

1 For a fuller explanation of nutrients, see Chapters VI and XXIV.

From

these raw food materials, organic foods are manufactured in the body of the plant.

BOOKS FOR REFERENCE

ELEMENTARY

Sharpe, A Laboratory Manual for the Solution of Problems in Biology. American
Book Company.

Avery-Sinnott, First Lessons in Physical Science. American Book Company.
Eddy, Experimental Physiology and Anatomy. American Book Company.

Snyder, The Chemistry of Plant and Animal Life. The Macmillan Company.

ADVANCED

Coulter, Barnes, and Cowles, A Textbook of Botany, Part II. American Book Company.

Foster, A Textbook of Physiology. The Macmillan Company.

Green, Vegetable Physiology. J. and A. Churchill.

Sedgwick and Wilson, General Biology. Henry Holt and Company.

III. THE FUNCTIONS AND COMPOSITION OF LIVING

THINGS

Problem IV. An introduction to the nature and work of living organisms. (Laboratory Manual, Prob. IV.)

(a) A living plant.

(b) Aliving insect.

A Living Plant and a Living Animal Compared.-A walk into the fields or any vacant lot on a day in the early fall will give us firsthand acquaintance with many common plants which, because of

A Weed. Notice the un

their ability to grow under sometimes unfavorable conditions, are called weeds. Such plants- the dandelion, butter and eggs, the shepherd's purse-are particularly well fitted by nature to produce many of their kind, and by this means drive out other plants which cannot do this so well. On these or other plants we find feeding several kinds of animals, usually insects.

If we attempt to compare, for example, a grasshopper with the plant on which it feeds, we see several points of likeness and difference at once. Both plant and insect are made up of parts, each of which, as the stem of the plant or the leg of the favorable habitat. Pho- insect, appears to be distinct, but which is tograph by W. A. Barbour. a part of the whole living plant or animal. Each part of the living plant or animal which has a separate work to do is called an organ. Thus plants and animals are spoken of as living organisms. Functions of the Parts of a Plant.

We are all familiar with the parts of a plant, the root, stem, leaves, flowers, and fruit.

But we may not know so much about their uses to the plant. Each of these structures differs from every other part, and each has a separate work or function to perform for the plant. The root holds the plant firmly in the ground and takes in water and mineral matter from the soil; the stem holds the leaves up to the light and acts as a pathway for fluids between the root and leaves; the leaves, under certain conditions, manufacture food for the plant and breathe; the flowers form the fruits; the fruits hold the seeds, which in turn hold young plants which are capable of reproducing adult plants of the same kind.

If

E

Section through the blade of a leaf, as seen under the compound microscope; S, air spaces, which communicate with the outside air; V, vein in cross section; ST, breathing hole; E, outer layer of cells; P, green cells.

The Functions of an Animal. we examine the grasshopper more carefully, we find that it has a head, a jointed body composed of a middle and a hind part, three pairs of jointed legs, and two pairs of wings. Obviously, the wings and legs are used for movement; a careful watching of the hind part of the animal shows us that breathing movements are taking place; a bit of grass placed before it may be eaten, the tiny black jaws biting little pieces out of the grass. If disturbed, the insect hops away, and if we try to get it, it jumps or flies away, evidently seeing us before we can grasp it. Hundreds of little grasshoppers indicate that the grasshopper can reproduce its own kind, but in other respects the animal seems quite unlike the plant. The animal moves, breathes, feeds, and has sensation, while apparently the plant does none of these. It will be the purpose of later chapters to prove that the functions of plants and animals are in many respects similar and that both plants and animals breathe, feed, and reproduce.

Organs. If we look carefully at the organ of a plant called a leaf, we find that the materials of which it is composed do not appear to be everywhere the same. The leaf is much thinner and more delicate in some parts than in others. Holding the flat, ex

panded blade away from the branch is a little stalk, the petiole, which extends into the blade of the leaf. Here it splits up into a network of tiny veins which evidently form a framework for the flat blade somewhat as the sticks of a kite hold the paper in place. If we examine under the compound microscope a thin section cut across the leaf, we shall find that the veins as well as the other parts are made up of many tiny boxlike units of various sizes and shapes. These smallest units of building material of the plant or animal disclosed by the compound microscope are called cells. The organs of a plant or animal are built of these tiny structures.

Tissues. The cells which form certain parts of the veins, the flat blade, or other portions of the plant, are often found in groups or collections, the cells of which are more or less alike in size and shape. Such a collection of cells is called a tissue. Examples of tissues are the cells covering the outside of the human body, the muscle cells, which collectively allow of movement, bony tissues which form the framework to which the muscles are attached, and many others.

Adaptations of Structure to Function. If I look at my hand as I write, I notice that the fingers of my right hand grasp the pen firmly; that because of the several joints in the fingers, the wrist, and forearm, free movement can be given to the hand when the muscles attached to the bones move it. The hand is capable of a great number of complicated and delicate movements, most of them associated with the work of grasping objects. Because of the peculiar fitness in the structure of the hand for this work, we say that it is adapted to this, its function, that is, grasping objects. Each organ of the plant is fitted or adapted in some way to do certain kinds of work. It is the object of the chapters following to point out how the parts of a plant or animal are adapted to their various functions.

Problem V. The structure and general properties of living matter. (Laboratory Manual, Prob. V.)

To the Teacher. - Any simple plant or animal tissue can be used to demonstrate the cell. Epidermal cells may be stripped from the body of the frog or obtained by scraping the inside of one's mouth. The thin skin from