2 TEXTBOOK OF PLANT PHYSIOLOGY 



based particularly on the physicochemical sciences. When a 

 detailed investigation in physiology is undertaken, the field of 

 biology is usually left behind and the realm of physics and chem- 

 istry is entered. In the course of such a study difficulties of two 

 kinds are encountered. In the first place, life activities may be 

 characterized by such complexities that it is very difficult to sub- 

 divide them completely into their physiological components. 

 And, considering the present state of our knowledge, it is often 

 impossible to subject them entirely to the laws of physics and of 

 chemistry. This is particularly true of many problems pertaining 

 to the physiology of growth and reproduction. Secondly, some 

 of the physicochemical sciences that are most closely connected 

 with physiology, as, for instance, physical and especially colloidal 

 chemistry, are as yet insufficiently developed. 



The first type of difficulties has prompted certain investigators 

 to take a pessimistic attitude toward the possibility of ever carry- 

 ing this analysis to an ultimate end. It has led them to believe 

 that in living organisms there exists a certain " something" which 

 cannot be subjected to a physicochemical analysis. This "some- 

 thing" was once defined as a "vital force," and this explanation 

 has become known by the name "vitalism." The theory of vital- 

 ism is opposed to the mechanistic conception of life, which regards 

 the organism as a physicochemical mechanism of very complex 

 character, to be sure, but nevertheless fundamentally not differing 

 from non-living things. 



The adherents of the mechanistic point of view, to which 

 belong the majority of the present-day physiologists, are con- 

 vinced that eventually it will be possible to analyze the most com- 

 plex vital phenomena into their physicochemical components. 

 The differences of opinion that exist between the vitalists and the 

 mechanists, of course, finally will be ironed out. But this may 

 happen only in the distant future when all the organic functions 

 become completely clear. One must emphasize the fact that the 

 history of plant physiology seems to be a history of victories gained 

 for the mechanistic theory. Moreover, the mechanistic principles 

 are far more helpful as working hypotheses to the investigators, 

 since they call for a thorough analysis of all phenomena. 



Our present incomplete knowledge of the adjoining fields of 

 physics and chemistry does not necessarily prevent physiologists 

 from carrying on their work. There are many instances on 

 record where physiologists, being prompted by the requirements of 



