PART II PHYSIOLOGY 



INTRODUCTION 



THE relation between the form and structure of a plant and its behavior 

 is very intimate and to a large extent reciprocal. Form and structure 

 in general determine behavior, and behavior, especially as it is itself 

 controlled by external agents, to a great degree determines form and 

 structure. It is not possible at present to discover all these reciprocal 

 relations, much less to describe them in terms of physics and chemistry. 

 Nor is the behavior of plants sufficiently known to be explained in these 

 terms. 



Morphology, concerned with form and structure, is particularly in- 

 terested in how each plant comes to be what it is in the short history 

 of its own life (ontogeny), and also seeks to form a conception of how 

 plants have come to be what they are in the long course of their history 

 since they began to develop on the earth (phylogeny). The former 

 topic is clearly open to experimental study and constitutes the field of 

 experimental morphology. But the latter is much less open to experi- 

 ment ; scarcely at all, indeed, except for the determination of the laws 

 of heredity, a field which has been called " experimental evolution." 

 Obviously such experiments, whether in the field or laboratory, cannot be 

 wisely planned or executed without a thorough knowledge of plant 

 physiology. 



A wide range of facts is open also to mere observation, because the 

 ordinary changes in climate and soil, some of which are produced by 

 other plants and animals, affect the form and structure of plants. This 

 field is part of that distinguished from physiology proper as Ecology 

 (Part III). Naturally even the most careful observations need to be 

 confirmed or corrected by experiments. Thus this portion of ecology 

 and experimental morphology are mutually related, and both really 

 form a part of physiology in the broadest sense, and depend upon it. 

 Physiology, in its turn, seeking to expound the phenomena of plant life 

 in terms of matter and force, depends upon the data of chemistry and 



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