EXTERNAL INFLUENCES 511 



may be suppressed by heat, provided the maximal temperature for the 

 formation of these substances lies below the critical point for the entire 

 plant. Inhibition of this character can, however, be exercised only upon 

 those processes which are not directly essential for the maintenance of 

 vital activity. 



Since the main features of this discussion have already been given, 

 it is only necessary to mention here a few general considerations such 

 as have not previously been dealt with. Nutritive substances not only 

 provide a supply of energy and material for constructive purposes, but also 

 act as stimuli of varied and frequently extremely intricate character. Many 

 non-nutritive substances may excite or depress either the general vital 

 activity or special functions only. Temperature may also act in different 

 ways, as may light also, which often constitutes a stimulus, and may 

 either awaken a dormant activity, or provide the energy necessary for its 

 performance. 



A change in the external conditions causes the general vital activity 

 to assume a new condition of equilibrium which is maintained so long as the 

 internal and external conditions suffer no further change. It can, however, 

 only be determined by direct experiment in each special case whether for 

 any metabolic product the balance is maintained by the equality between 

 production and consumption, or by the cessation of the former. Every 

 compound which is continually drawn into metabolism must be as continually 

 replaced, and hence further production must ultimately cease whenever any 

 product remains intact in the plant. This is the case, for example, with 

 pigments, poisons, enzymes, &c., the formation of which may under special 

 circumstances entirely cease, while on the other hand a plastic substance 

 such as glycerine may under certain conditions remain completely intact 

 (Sect. 67). The self-regulation of metabolism probably frequently in- 

 volves interactions of a similar character, and it is not impossible that the 

 starch-grains, cell-walls, &c. of living cells, which seem to be permanent 

 structures, are really subject to continual dissolution and regeneration. 

 Similarly the maintenance of the acidity of the cell-sap may be due either 

 to the balance between production and decomposition, or to the self- 

 regulatory cessation of production as soon as a certain amount has 

 accumulated (Sect. 86). The latter is probably the case as regards the 

 salts of organic acids, the regulatory formation excited by a diminution 

 maintaining a constant turgidity during growth. As regards the latter, 

 moreover, various facts show that the potential powers of growth are 

 by no means always fully exercised, and may indeed under certain conditions 

 remain permanently dormant (Sect. 4). 



The new condition of equilibrium produced by a marked change of 

 temperature is apparently assumed rapidly and without causing any great 

 disturbance. When, however, the partial pressure of oxygen is rapidly 



