8 INTRODUCTION 



subservient to this function are less obscured by the other vital processes 

 and functions, and therefore can be more clearly perceived. Consequently 

 the study of such specialized functions is of the utmost importance, and 

 may throw light upon the nature of the vital mechanism. It is of almost 

 equal importance in this respect that we should recognize that, even in 

 unicellular organisms, powers and properties of very different degree of 

 physiological differentiation may be present. 



SECTION 2. The Aim of Physiology. 



In the previous pages the aims and purposes of Physiology have been 

 briefly indicated. Regarding them from a general point of view, these 

 are to study the nature of all vital phenomena in such a manner, that 

 by referring them to their immediate causes, and subsequently tracing 

 them to their ultimate origin, we may arrive at a complete knowledge 

 of their importance in the life of the organism. 



A physiological problem is connected with every manifestation of life. 

 Many phenomena, however, the morphological aspects of which are familar 

 to us, are imperfectly understood from a physiological point of view, and 

 consequently anything like a complete and perfect treatise on Physiology is at 

 present impossible. We cannot, however, leave unmentioned those processes 

 concerning which a certain amount of physiological knowledge has already 

 been obtained, and hence the present volume gives a general account of the 

 sources and means by which plants obtain food and energy, the modifications 

 and changes which the food and energy thus obtained may undergo, and the 

 manifestations of energy which find visible expression in plant life and growth. 



It is the province of general Physiology to investigate what are the 

 essential factors which influence, or actually cause, the definite phenomena 

 that come under our observation, as well as the precise nature of the relation- 

 ships between them. When once these are clearly established, some idea of 

 the relative importance of the various phenomena may be obtained, which 

 in the absence of this preliminary knowledge is impossible. A textbook 

 of general Physics treats of the properties of steam and electricity, but 

 is not called upon to describe every machine that the wit of man has ever 

 devised ; similarly, it is not, nor can it ever be, the task of general Physiology 

 to describe in detail the peculiarities of single plants or even of special 

 groups of plants. This is the aim of a monographic study. It would 

 consequently be beyond the scope of this book to give a special account of 

 the details of the metabolism of Bacteria, or of the processes of fertilization 

 and development. These and other phenomena will be dealt with only so 

 far as they are connected with, and illustrate, the essenthl principles of 

 general Physiology. 



Since all our physiological knowledge must necessarily be based upon 



