CHAPTER I. GENERAL PHYSIOLOGY. 667 



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 produc- 

 tion of new individuals resembling itself. Hence a distinction may 

 at once be drawn between the nutritive and the reproductive pro- 

 perties 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 manifesta- 

 tion 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 

 be spontaneously performed by various parts in virtue of the auto- 

 matism of the protoplasm : the motile property of protoplasm may 

 be termed motility. 



The nutritive property of protoplasm requires, however, further 

 analysis. First, 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. Secondly, from the food 

 absorbed, protoplasm must ultimately be formed ; the building up 

 of protoplasm out of the food is termed assimilation, and the pro- 

 perty by means of which this function is performed is termed the 

 metabolic property of protoplasm. 



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. 



Stating these points in a more general form, it is to be clearly 

 apprehended that there are two sets of chemical processes con- 

 tinually and simultaneously going on in living protoplasm. 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 decom- 

 position of more complex ones. The former are designated the 

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

 plasm ; 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 



