CHAPTER II

THE ORGANIC COMPONENTS OF PLANTS

From the standpoint of their ability to synthetize synergic foods (see page 2) from inorganic raw materials, plants may be divided into two types; namely, the _autotrophic_, or self-nourishing, plants, and the _heterotrophic_ plants.

Strictly speaking, only those plants whose every cell contains chlorophyll are entirely self-nourishing; and some parts, or organs, of almost any autotrophic plant are dependent upon the active green cells of other parts of the plant for their synergic food. Furthermore, if the term is used in a very wide sense, green plants are more than self-nourishing, they really nourish all living things. But the general significance of the term "autotrophic plants" is apparent.

"Heterotrophic plants" must, of necessity, get food, either directly or indirectly, from some other plant which can synthetize synergic foods or, in a few cases, from animal organic matter. If they do this by feeding upon the organic compounds of other living organisms, they are known as "parasites"; while if they secure their organic food from the tissues or debris of dead organisms, they are called "saprophytes." The heterotrophic plants are chiefly the bacteria and fungi; although a few seed-plants are devoid of chlorophyll or have nutritive habits similar to those of the non-green plants, and a few species are semi-parasitic or semi-saprophytic.

It is obvious that the metabolic processes of the autotrophic plants are very different from those of the heterotrophic type of plants. These differences constitute a most interesting field of study for plant physiologists. But the nature of the chemical compounds themselves and of the chemical changes involved in their transformations is not radically different in the two types of plants, the essential difference being in the preponderance of one kind of activities, or chemical reactions, over another in bringing about the metabolic processes which are characteristic of each particular species. Hence, it does not seem necessary, or desirable, in this study of the chemistry of plant growth, to present as detailed a consideration of the differences in metabolic activity of the different types of plants as complete accuracy of statement in all cases might demand. We will, instead, discuss the organic chemical components of plant tissues and the reactions which they undergo, using the more common type of autotrophic plants as the illustrative material in most cases.

Hence, it will be understood that in all the following discussions of plant

## activities, except where specific exceptions are definitely mentioned, it

is the green, or autotrophic, plants to which reference is made in each case.

From the standpoint of the sum total of its activities, a green plant is essentially an absorber of solar energy and a synthetizer of organic substances. Each individual autotrophic plant takes up certain amounts of the anergic foods which are discussed in the preceding chapter and manufactures from them a great variety of complex organic compounds, using the energy of the sun's rays, absorbed by chlorophyll, as the source for the energy necessary to accomplish these synthetic reactions. The ultimate object of these processes is to produce seeds, each containing an embryo and a sufficient supply of food for the young plant of the next generation to use until it has developed its own synthetic organs; or (in the case of perennials) to store up reserve food materials with which to start off new growth after a period of rest and often of defoliation. To be sure, animals and men often interfere with the completion of the life cycle of the plant, and utilize the seeds or stored food material for their own nutrition, but this is a biological relation which has no influence upon the nature of the plant's own activities.

Since all of these synthetic reactions must go on at ordinary temperatures,

## active catalyzers are necessary. These the plant provides in the form of

enzymes (see Chapter XIV ) which are always present in active plant protoplasm. Proper conditions for rapid chemical action are further assured by the colloidal nature (see