INTRODUCTION 



So IMPORTANT a part do diffusion and osmotic pressure 

 seem to play in the vital processes of plants, that it is well- 

 nigh impossible to consider any phase of vegetable physiology 

 without some reference to these subjects. It is obviously 

 not to the point, however, to attempt here a discussion of 

 every phenomenon in plant life into which they enter. 

 Rather will attention be directed to certain groups of 

 phenomena wherein diffusion and osmotic pressure seem to 

 be fundamental factors. Thus, it is hoped, may be formed 

 a general conception of the trend which modern study is 

 taking along these lines. 



Of the four following chapters, the first three have to do 

 with osmotic pressure as an internal factor in the life of the 

 plant ; in them are considered the most important effects of 

 the development of diffusion tensions within the plant body. 

 In the last chapter are brought together the responses of the 

 organism to variations in the osmotic pressure of the sur- 

 rounding medium. Such division of the subject is merely 

 expedient; it is purely artificial, for the organism and its 

 surrounding medium are physically almost as truly continu- 

 ous as are a mass of ice and the water in which it floats. 

 Also a fact which is often apparently lost sight of every 

 portion of the plant body is a portion of the environment of 

 every other portion. This is of fundamental importance, 

 especially in the physiology of multicellular forms. How- 

 ever, the plant body is a fairly definite thing, and in the 

 present state of our knowledge the above classification of 

 environmental factors is perhaps as good as any other. 



In the following pages authors are cited for the most 



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