138 ENVIRONMENTAL CONDITIONS. 



V. CHEMICAL CONDITIONS. 



1. REQUIREMENT OF MATERIAL WITHIN THE PLANT. 



At the beginning of this treatment of the main categories of environ- 

 mental conditions we discussed the water-relation of plants. Water 

 was given a place by itself in our series because its importance in the 

 organism, as has been seen, appears to depend more upon its solvent 

 powers and power to be imbibed in the plant colloids than upon its 

 chemical influence. But it has been pointed out that water also 

 acts chemically in the plant, being one of the two substances chemically 

 transformed in photosynthesis and likewise one of the two products of 

 the process of respiration. It is probably chemically important in 

 other ways, certainly playing an essential part in many processes of 

 hydration polymerization, hydrolysis, etc. 



Of course there are innumerable other substances, besides water, 

 tha,t take part chemically in plant activities. We are apt to think 

 first of the three great groups of compounds that have been called 

 foods— the carbohydrates, fats, and proteins. These are apparently 

 all essential to vital activity and even to the mere retention of life 

 in the most dormant phases. Besides these there are a large number 

 of substances of a more or less complex nature that are sometimes con- 

 sidered as foods and sometimes not. Here may be mentioned gluco- 

 sides, alkaloids, various lipoids like the cholesterins, phytostearins, 

 etc. Also, substances of importance in certain of the simpler com- 

 ponent processes of vital activity, like chlorophyll; the various 

 enzymes— still of questionable nature— etc., may be classified here. 

 Finally, in order that life may occur, there must be in the tissues a 

 number of inorganic salts and their ionized products. These are 

 not classified as foods by physiologists, although conservative agri- 

 culturists are still prone to speak of them as ''plant-foods." They 

 might better be termed auxiliary substances until some such time 

 as the word ''foods" may be dropped from physiology. The condi- 

 tions in different protoplasms, in different plants, in different de- 

 velopmental phases of the same plant and different parts of the same 

 individual, are, however, so extremely varied that there seems little 

 ultimate value in attempting to classify the various materials that 

 are essential to plant activity. It is certainly far simpler, and prob- 

 ably as satisfactory in every way, to consider merely the material 

 conditions of life, classifying the different substances on purely chemical 

 grounds. We need here merely emphasize the well-known point that 

 one of the prime conditions for organic life is the presence in the 

 organism of innumerable kinds of chemical compounds. 



Since all vital activity must be regarded as material change of some 

 sort, it is clear that the quantitative and qualitative rektions between 

 these many substances must be continually changing; the substances 



