CHAPTER XXVI

LEAVES

Structure of leaves. It is necessary to know something about the structure of leaves in order to understand the method by which they carry on the processes which make them such important parts of plants. You

already know that a leaf has a broad part of green color, the blade. Most leaves also have a

stem-like part, the petiole. Sometimes two leaf-like

parts, stipules, are

present at the base

of the petiole. The
blade may take al-
most any conceiv-

Mid
Rib

Vein

Blade

called veins. These veins are continuous with tubes passing through the petiole and down through the stem of the plant into the root. Veins

Fig. 77. Leaf of lily-of-thevalley, parallelveined.

usually present a netted appearance, but in some leaves, such as those of lilies and grasses, they run in somewhat parallel lines.

When examined under a microscope, each leaf is found to be composed of an almost countless number of cells, which vary in structure and in use. The outer surface, or epidermis, of the leaf is composed of large irregular-shaped cells. The interior of the leaf is filled between the veins with loosely arranged cells forming what is known as the mesophyll. The epidermis,

structure.

although frequently very thin, serves as a protection to the mesophyll, which is easily crushed and dies quickly when the epidermis is removed. In the epidermis, on the under surface of the leaf, there are a number of tiny oval openings. These are called

stomates. (The Greek word stoma means mouth.) On each side of a stomate there is a kidney-shaped cell called a guard cell. The shape of these cells can be

gc

Fig. 79. Epidermis of а leaf of geranium showing stomates; c, cell; P, opening of stomate; gc, guard cell.

changed and in so doing the openings into the leaf can be made smaller or larger. It is not exactly clear of just what service this is to the leaf. There are thousands of stomates on each leaf, usually only on the

under surface. Gases pass in and out of the leaf through the

A crossstomates. section of a leaf shows

that, immediately below the epidermis, there is a row of green cells closely packed together. Below these are the large, green, loosely-arranged cells of the mesophyll. The stomates open directly into the spaces, air chambers, formed between these cells.

Chlorophyll. Some things in nature are so common that we do not stop to study them; we take them for granted. We know that most plants have a great many leaves and that these leaves are green. We perhaps have never considered the nature or importance of this green color. Inside of all the cells of the leaves,

except those of the epidermis, there are a number of green colored bodies called chloroplasts. They are principally composed of protoplasm by which a green substance, chlorophyll, is manufactured. (Chloron, green; phyllon, leaf.) The chlorophyll gives the green color to the chloroplast. It is upon the presence of this substance that the most important work of the plant depends, and upon that work all life, including our own, depends. Think of the significance of this fact.

Food making. We have already stated that a plant uses as a part of the raw materials, from which to make its food, carbon dioxide and water. Out of these inorganic materials leaves can manufacture starch; this is their principal function. The water is obtained from the soil and passes up through bundles of tubes from the roots to the veins of the leaves. Carbon dioxide from the air enters through the stomates. The carbon dioxide and water are then combined to form starch or sugar. Starch and sugar belong to a group of foods known as the carbohydrates, which are composed of carbon, hydrogen, and oxygen. They always contain twice as much hydrogen as oxygen. To form carbohydrates from the raw materials we know that chloroplasts and light are essential. By means of protoplasm and chlorophyll, chloroplasts absorb energy from the sun's rays. With this energy, the chloroplasts are able to break the compounds, water and carbon dioxide, into carbon, hydrogen, and oxygen, the elements of which they are composed. These three ele

Fig. 80. Experiment to show that oxygen is given off

ments then immediately unite and finally form sugar and starch. The exact chemical processes which this involves are not completely known. Carbohydrates contain twice as much hydrogen as oxygen. Carbon dioxide (CO2) and water (H2O) contain only two parts of hydrogen for every three of oxygen. Consequently there is an excess of oxygen in this process. This oxygen is given off as a by

by green plants in product. The process of starch-making may be expressed with the follow

the sunlight.

ing chemical equation:

N (6CO2) + N (5H2O) = N (C ̧H100) + N (602)

5

Carbon dioxide + water = starch + oxygen On bright days starch is formed very rapidly. During the night this food is transformed into soluble forms and then moves from the leaf to other parts of the plant. Much of this is stored away in the form of starch in such parts of plants as the seeds of cereals, grains, fruits, potato tubers, etc. From these all animals derive food. Upon this starch-making process of the green plants all life is dependent for the ultimate source of food. Carbohydrates make up a very large part of the food of all animals. Indirectly the meat which we eat is derived from these carbohydrates made by plants. Beef, for example, is procured directly from