The Chemistry of Plant Life

Chapter Xv--the Colloidal Condition

"Colloids" and "crystalloids"; the colloidal condition a dispersion phenomenon; nomenclature and classification; conditions necessary to the formation of sols; gel-formation; general properties of colloidal solutions; suspensoids and emulsoids; adsorption; catalysis affected by the colloidal condition; industrial applications of colloidal phenomena; natural colloidal phenomena; references 202-220

Chapter Xvi--the Physical Chemistry of Protoplasm

Heterogeneous structure of protoplasm; protoplasm a colloidal gel; water; salts; osmotic pressure; surface boundary phenomena; electrical phenomena; acidity and alkalinity; summary; vital phenomena as chemical and physical changes; references 221-238

Chapter Xvii--Hormones, Auximones, Vitamines, and Toxins

External and internal stimulants; hormones; vitamines; auximones, toxins 239-248

CHAPTER XVIII--ADAPTATIONS

General discussion; adaptations, accommodations, and adjustments; chromatic adaptations; morphological adaptations; accommodations; concluding statements 249-258

INDEX 259-268

INTRODUCTION

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 students had only vague notions of the actual structure of a living organism. Beginning with the earliest idea that a plant or an animal exists as a unit organism, to be studied as such, biological science progressed, first to the recognition and study of the individual organs which are contained within the organism; then to the tissues which make up these organs; then (with the coming into use of the microscope as an aid to these investigations) to the cells of which the tissues are composed; then to the protoplasm which constitutes the cell contents; and finally to the doctrine of organic evolution as the explanation of the genealogy of plants and animals, and the study of the relation of the principles of the physical sciences to the evolutionary process. The ultimate material into which organisms are resolved by this process of biological analysis is the cell protoplasm. But protoplasm is itself made up of a complex system of definite chemical compounds, which react and interact according to the laws of physical science. Hence, any study of the chemistry of plant growth is essentially a study of the chemical and physical changes which take place in the cell protoplasm.

Protoplasm differs from non-living matter in three respects. These are (1) its chemical composition; (2) its power of waste and repair and of growth; and (3) its reproductive power. From the standpoint of chemical composition, protoplasm is the most complex material in the universe. It not only contains a greater variety of chemical elements, united into molecules of enormous size and complexity, but also a greater variety of definite chemical compounds than exist in any other known mixture, either mineral or organic in type. One of the first problems in the study of protoplasm is, therefore, to bring this great variety of complex compounds into some orderly classification and to become familiar with their compositions and properties. Again, living matter is continually undergoing a process of breaking down as a result of its energetic activities and of simultaneously making good this loss by the manufacture of new protoplasm out of simple food materials. It also has the power of growth by the production of surplus protoplasm which fills new cells, which in turn produce new tissues and so increase the size and weight of individual organs and of the organism as a whole. Hence, a second field of study includes the chemical changes whereby new protoplasm and new tissue-building material are elaborated. Finally, living material not only repairs its own waste and produces new material of like character to it, but it also produces new masses of living matter, which when detached from the parent mass, eventually begin a separate existence and growth. Furthermore, the plant organism has acquired, by the process of evolution, the ability not only to produce an embryo for a successive generation but also to store up, in the tissues adjacent to it, reserve food material for the use of the young seedling until it shall have developed the ability to absorb and make use of its own external sources of food material. So that, finally, every study of plant chemistry must take into consideration the stored food material and the germinative process whereby this becomes available to the new organism of the next generation. Also, the chemistry of fertilization of the ovum, so that a new embryo will be produced, and the other stimuli which serve to induce the growth phenomena, must be brought under observation and study.

A further step in the development of biological science has been to separate the study of living things into the two sciences of botany and zoology. From the standpoint of the chemistry of the processes involved this segregation is unfortunate. It has resulted in the devotion of most of the study which has been given to life processes and living things to animal chemistry, or "physiological chemistry." As a consequence, biochemistry, which deals with the living processes of both plants and animals, is yet in its infancy; while phytochemistry is almost a new science, yet its relation to the study of plants can scarcely be less vital than is that of physiological chemistry to studies of animal life.

The common conception that plant life and animal life are antithetical or complementary to each other has much to justify it. Animals breathe in oxygen and exhale carbon dioxide; while plants use the carbon dioxide of the air as a part of the raw material for photosynthesis and exhale oxygen. Plants absorb simple gases and mineral compounds as raw food materials and build these up into complex carbohydrates, proteins, fats, etc.; while animals use these complex compounds of plant origin as food, transforming parts of them into various other forms of structural material, but in the end breaking them down again into the simple gases and mineral compounds, which are expelled from the body through the excretory organs. Thus it would seem that the study of the chemistry of plant life and of animal life must necessarily deal with opposite types of phenomena.

But one cannot advance far into the study of the biochemistry of plants and animals before he discovers marked similarities in the chemical principles involved. Many of the compounds are identical in structure, undergo similar changes, and are acted upon by similar catalysts. Plant cells exhibit respiratory activities, using oxygen and giving off carbon dioxide, in exactly the same way that animal organisms do. The constructive photosynthetic processes of green plants are regulated and controlled by a pigment, chlorophyll, which is almost identical with the blood pigment, hæmatin, which regulates the vital activities in the animal organism, differing from the latter only in the mineral element which links the characteristic structural units together in the molecule. Many other points of similarity in the chemistry of the life processes of plants and animals will become apparent as the study progresses. It is sufficient now to call attention to the fact that these vital processes, in either plants or animals, are essentially chemical in character, and subject to study by the usual methods of biochemical investigations.

The protoplasm of the cell is the laboratory in which all the changes which constitute the vital activities of the plant take place. All of the processes which constitute these activities--assimilation, translocation, metabolism, and respiration--involve definite chemical changes. In so far as it is possible to study each of these activities independently of the others, they have been found to obey the ordinary laws of chemical reactions. Thus, the effect of the variations in intensity of light upon photosynthesis causes increase in the rate of this activity which may be represented by the ordinary responses of reaction velocities to external stimuli. Similarly, the effect of rises in temperature upon the rate of assimilation and upon respiration are precisely the same as their effect upon the velocity of any ordinary chemical reaction. Within certain definite ranges of temperature, the same statement holds true with reference to the rate of growth of the plant, although the range of temperature within which protoplasm lives and maintains its delicate adjustment to the four vital processes of life is limited; beyond a certain point, further rise in temperature does not produce more growth but rather throws the protoplasmic adjustment out of balance and growth either slows up markedly or stops altogether.

Hence, we may say that the methods by which the plant machine (protoplasm) accomplishes its results are essentially and definitely chemical in character and may be studied purely from the standpoint of chemical reactions, but the maintenance of the machine itself in proper working order is a vital phenomenon which is largely dependent upon the external environmental conditions under which the plant exists. A study of the phenomena resulting from the colloidal condition of matter is throwing a flood of light upon the mechanism by which protoplasm accomplishes its control of vital activities. But we are, as yet, a long way from a complete understanding of how colloidal protoplasm acquires and maintains its unique ability of self-regulation of the conditions necessary to preserve its colloidal properties and of how it elaborates the enzymes which control the velocity of the chemical reactions which take place within the protoplasm itself and which constitute the various processes of vital activity.

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