Copyright 1921 , BY THE McGRAW-HILL BOOK COMPANY,  Inc....

The author has had in mind a two-fold purpose in the preparation of this book. First, it is hoped that it may serve as a text or reference book for collegiate students of plant science who are seeking a proper foundation upon which to build a scientific knowledge of how plants grow. The late Dr. Charles E. Bessey, to whom I owe the beginning of my interest in plant life, once said to me: "The trouble with our present knowledge of plant science is that we have had very few chemists who knew any b

The history of biological science shows that the conceptions which men have held concerning the nature of plant and animal growth have undergone a series of revolutionary changes as the technique of, and facilities for, scientific study have developed and improved. For a long time, it was thought that life processes were essentially different in character than those which take place in inanimate matter, and that the physical sciences had nothing to do with living changes. Then, too, earlier stud

There is some confusion in the use of the terms "nutrient," "plant food," etc., as applied to the nutrition and growth of plants. Strictly speaking, these terms ought probably to be limited in their application to the organized compounds within the plant which it uses as sources of energy and of metabolizable material for the development of new cells and organs during its growth. Botanists quite commonly use the terms in this way. But students of the problems involved in the relation of soil ele

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. Furthermo

Photosynthesis is the process whereby chlorophyll-containing plants, in the presence of sunlight, synthetize organic compounds from water and carbon dioxide. The end-product of photosynthesis is always a carbohydrate. Chemical compounds belonging to other groups, mentioned in the preceding chapter, are synthetized by plants from the carbohydrates and simple raw materials; but in such cases the energy used is not solar energy and the process is not photosynthesis. Under the ordinary conditions of

These substances comprise an exceedingly important group of compounds, the members of which constitute the major proportion of the dry matter of plants. The name "carbohydrate" indicates the fact that these compounds contain only carbon, hydrogen, and oxygen, the last two elements usually being present in the same proportions as in water. As a rule, natural carbohydrates contain six, or some multiple of six, carbon atoms and the same number of oxygen atoms less one for each additional group of s

These substances constitute a group of compounds which are very similar to the polysaccharide carbohydrates in composition and constitution, but which serve entirely different purposes in the plant. As a class, they are condensation products of pentoses, known as pentosans and having the formula (C 5 H 8 O 4 ) n , or hexosans having the formula (C 6 H 10 O 5 ) n , or combined pentosan-hexosans. In general, these compounds make up the skeleton, or structural framework material, of the plant, in c

Strictly speaking, the term glucoside should be applied only to such compounds as contain glucose as the characteristic basic group. But in common usage, it refers to any compound which, when hydrolyzed, yields a sugar as one of the products of the hydrolysis. In all the natural glucosides which occur in plant tissues, the other organic constituent, which is represented by the R in the formula for glucosides (R·C 6 H 11 O 5 , or R·(CHOH) 5 CHO) is some aromatic group, or closed-ring benzene deri

Using the term in its general application to a group of substances having similar chemical and physical properties, rather than in its limited application to a single definite chemical compound known commercially as "tannin," the tannins are a special group of plant substances, mostly glucosides, which have the following characteristic properties. First, they are non-crystalline [4] substances, which form colloidal solutions with water, which have an acid reaction and a sharp astringent taste. S

Practically all plant structures contain pigments. These may be considered as of two types: ( a ) the vegetative pigments, which have a definite energy-absorbing rôle in the metabolic processes of the tissues which contain them, and ( b ) the ornamental pigments. It is probable that the same chemical compound may serve in either one of these capacities under different conditions, but, in general, it is possible to assign either a definite vegetative, or physiological, use, or else a simple ornam

Organic acids, either in free form, or partially neutralized with calcium, potassium, or sodium, forming acid salts, or combined with various alcohols in the form of esters, are widely distributed in plants. They occur in largest proportions in the fleshy tissues of fruits and vegetables, where they are largely responsible for the flavors which make these products attractive as food for men and animals. But organic acids and their salts are also found in the sap of all plants, and undoubtedly pl

Included in this group are several different kinds of compounds which have similar physical properties, and which, in general, belong to the type of organic compounds known as esters, i.e., alcoholic salts of organic acids. The terms "oil," "fat," and "wax," are generally applied more or less indiscriminately to any substance which has a greasy feeling to the touch and which does not mix with, but floats on, water. There are many oils which are of mineral origin which are entirely different in c

Included in this group are all those substances to which the characteristic odors of plants are due, along with others similar in structure and possessing characteristic resinous properties. They have no such uniformity in composition as is exhibited by the oils which are included among the fats and waxes; but belong to several widely different chemical groups. Furthermore, there is no sharp dividing line between the essential oils and certain esters of organic acids on the one hand and the fats

We come, now, to the consideration of the characteristically nitrogenous compounds of plants. None of the groups of compounds which have been considered thus far have, as a group, contained the element nitrogen. This element is present in the chlorophylls and in certain other pigments, but not as the characteristic constituent of the molecular structure of the group of compounds, nor do these compounds serve as the source of supply of nitrogen for the plant's needs. The characteristic nitrogen-c

The proteins are the most important group of organic components of plants. They constitute the active material of protoplasm, in which all of the chemical changes which go to make up the vital phenomena take place. Combined with the nucleic acids, they comprise the nucleus of the cell, which is the seat of the power of cell-division and, hence, of the growth of the organism. Germ-cells are composed almost exclusively of protein material. Hence, it is not an over-statement to say that proteins fu

The characteristic difference between the reactions of inorganic compounds and those of organic substances lies in the rapidity, or velocity, of the chemical changes involved. Speaking generally chemical reactions take place between substances which are in solution, so that they may come into sufficiently intimate contact that chemical action between them can take place. There are, of course, occasional examples of reactions between dry solids, such as the explosion of gunpowder, etc., but the g

Reference has frequently been made, in preceding chapters, to the fact that proteins, enzymes, lipoids, etc., exist in the protoplasm of plants and animals in the colloidal condition. The properties and uses of these compounds by plants depend so much upon this fact that, before proceeding to the consideration of the actual physical chemistry of protoplasm itself, it will be appropriate and profitable to give some attention to the nature and significance of the colloidal condition of matter and

Thus far, we have considered the chemical nature of the various groups of compounds which are found in the tissues of living organisms, laying emphasis upon those which are of plant origin. These compounds constitute the material, or machinery, of the cell, and their various transformations furnish the energy for its operation. We come now to a study of the mode of its operation, or the processes of vital phenomena. Our knowledge of these matters is not yet far enough advanced to permit a defini

Reference has frequently been made, in preceding chapters, to the effect of various stimulating or inhibiting agencies upon the physiological activities of plant protoplasm. In the main, these agencies are external to the plant and are either physical, such as changes of temperature, amount of light received, etc.; or chemical, such as variations in the salts received from the soil, or common anæsthetics applied to the plants by man. A plant grows normally under certain conditions to which it ha

Most of the discussions which have been presented in the preceding chapters have dealt with the types of compounds, the kinds of reactions, and the mechanism for the control of these, which are exhibited by plants under their normal conditions for development. The results of the evolutionary process have produced in the different species of plants certain fixed habits of growth and metabolism. So definitely fixed are these that in each particular species of plants each individual differs from ot