158 PART III. PHYSIOLOGY. [ 38 



vital functions are performed by the protoplasm, though the other 

 cell-contents and the cell-walls are not without their physiological 

 importance. With regard to the functions themselves, it is 

 apparent, in the first place, that the outcome of the physiological 

 activity of the plant is the maintenance of itself, and the production 

 of new individuals resembling itself. Hence a distinction may 

 at once be drawn between the nutritive and the reproductive 

 properties of protoplasm. Moreover, during its life, the plant 

 responds, in a more or less marked manner, to the action of external 

 forces, such as light, gravity, etc. This is a manifestation of 

 another property of the protoplasm, namely irritability or 

 sensitiveness. Very commonly the response to the action of the 

 external forces is of the nature of movement ; but movements may 

 also be performed spontaneously. 



It is clear that nutrition necessarily depends upon the absorption 

 of food from without ; hence the plant is capable of performing the 

 function of absorption. From the food absorbed, protoplasm is 

 ultimately formed; the building up of protoplasm out of the food is 

 termed assimilation, and the property by means of which this 

 function is performed is termed the metabolic property of proto- 

 plasm. But the metabolic processes going on in the protoplasm are 

 not only such as lead to its maintenance or increase in bulk ; on the 

 contrary, the protoplasm is continually undergoing decomposition. 

 It is to be clearly apprehended that there are two sets of chemical 

 processes continually and simultaneously going on in living proto- 

 plasm. Of these, which together constitute the metabolism of the 

 plant, one set includes those processes which lead to the formation 

 of more complex substances from simpler ones ; the other, those 

 processes which lead to the formation of simpler substances by the 

 decomposition of more complex ones. The former are designated 

 the constructive metabolism, or more shortly the anabolism, of the 

 protoplasm ; the latter are designated the destructive metabolism, 

 or the catabolism, of the protoplasm. It must also be clearly 

 understood that these two sets of processes affect not only the state 

 of the matter or substance of which the plant consists, but also the 

 state of the energy in the plant ; for the anabolism is accompanied 

 by a conversion of kinetic into potential or latent energy ; and the 

 catabolism, by a conversion of potential into kinetic energy. 



The products of metabolism may be classified as plastic products 

 and waste-products : the former are such as can be further worked 

 up in anabolism ; the latter are not so used, but are withdrawn 



