2 INTRODUCTION 



material becomes of value to the plant only when subjected to the con- 

 structive and destructive metabolism of the living organism. In the 

 course of these processes, the exact nature of which is determined by 

 the vital activity of the organism itself, the substances necessary for 

 growth are elaborated, while, at the same time, kinetic energy is supplied 

 for growth and movement, for internal and external work. 



In the absence of such metabolism, vital activity is impossible, and, 

 even in the adult plant, anabolic and katabolic changes are always taking 

 place so long as life remains l . In plants as well as in animals a large, 

 indeed often the greater, proportion of the nourishment absorbed serves 

 to supply the energy necessary for the maintenance of life, while a part 

 only is employed in building up the substance and tissues of the organism. 



Metabolism and vital activity are mutually interdependent ; the energy 

 set free by one metabolic process affords the necessary stimulus to further 

 metabolic activity, just as a fire, by heating the surrounding wood to 

 combustion point, ensures its own continuance. 



This correlation and mutual interdependence must also be borne in 

 mind when we attempt to distinguish between chemical and physical pro- 

 cesses exhibited by a living plant, and indeed the distinction between 

 these is purely arbitrary. Every chemical change necessarily involves 

 a redistribution of energy, and vice versa ; so that any profound study of 

 the phenomena of growth and movement must inevitably include that of the 

 chemical processes with which the former are inextricably associated. 



The same general laws hold good for all forms of life, including 

 bacteria. These latter organisms teach us that all the conditions for 

 active life exist in the smallest cell, and that the essential vital processes 

 can be carried on in the entire absence of external differentiation. The 

 development of external members and consequent internal tissue-differ- 

 entiation is correlated with a more or less complete division of labour 

 and function. Hence Physiology is primarily required to determine the 

 powers and possibilities of individual organs and cells and their various 

 interactions. 



Parts which are no longer living often perform useful functions. Dead 

 cells, air vessels, intercellular spaces, all of which are usually present in 

 the tissues of higher plants, have definite duties to perform, commonly 

 subsidiary to the functions of the living parts, or assisting in the per- 

 formance of them. D<$ad structural units often form the channels for the 

 passage of water and dissolved food materials, while air-spaces permit of 

 gaseous interchange, and allow the circulation of the indispensable oxygen 

 to the innermost tissues. These and similar processes are only of physio- 



1 [It must, however, be remembered that dormant life is possible in certain seeds, spores, 

 mosses, &c., in the total absence of all metabolic and respiratory activities.] 



