Section through the blade of a leaf. e, cells of the upper surface; d, cells of the lower surface; i, air spaces in the leaf; v, vein in cross sections; p, green cells.
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, expanded blade away from the branch is a little stalk, 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.
Several cells of Elodea, a water plant. chl., chlorophyll bodies; c.s., cell sap; c.w., cell wall; n., nucleus; p., protoplasm. The arrows show the direction of the protoplasmic movement.
Tissues.[2]—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.
A cell. ch., chromosomes; c.w., cell wall; n., nucleus; p., protoplasm.
Cells.—A cell may be defined as a tiny mass of living matter containing a nucleus, either living alone or forming a unit of the building material of a living thing. The living matter of which all cells are formed is known as protoplasm (formed from two Greek words meaning first form). If we examine under a compound microscope a small bit of the water plant Elodea, we see a number of structures resembling bricks in a wall. Each "brick," however, is really a plant cell bounded by a thin wall. If we look carefully, we can see that the material inside of this wall is slowly moving and is carrying around in its substance a number of little green bodies. This moving substance is living matter, the protoplasm of the cell. The green bodies (the chlorophyll bodies) we shall learn more about later; they are found only in plant cells. All plant and animal cells appear to be alike in the fact that every living cell possesses a structure known as the nucleus (pl. nuclei), which is found within the body of the cell. This nucleus is not easy to find in the cells of Elodea. Within the nucleus of all cells are found certain bodies called chromosomes. These chromosomes in a given plant or animal are always constant in number. These chromosomes are supposed to be the bearers of the qualities which we believe can be handed down from plant to plant and from animal to animal, in other words, the inheritable qualities which make the offspring like its parents.
How Cells form Others.—Cells grow to a certain size and then split into two new cells. In this process, which is of very great importance in the growth of both plants and animals, the nucleus divides first. The chromosomes also divide, each splitting lengthwise and the parts going in equal numbers to each of the two cells formed from the old cell. In this way the matter in the chromosomes is divided equally between the two new cells. Then the rest of the protoplasm separates, and two new cells are formed. This process is known as fission. It is the usual method of growth found in the tissues of plants and animals.
