xvi INTRODUCTION 



found to obey the ordinary laws of chemical reactions. Thus, 

 the effect of the variations in intensity of light upon photosyn- 

 thesis 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 main- 

 tains 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 phe- 

 nomena resulting from the colloidal condition of matter is throw- 

 ing 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 proto- 

 plasm 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. 



The object of this study of the chemistry of plant growth is 

 to acquire a knowledge of the constitution of the compounds 

 involved and of the conditions under which they will undergo the 

 chemical changes which, taken all together, constitute the vital 

 processes of cell protoplasm. 



