THE PLANT AND ITS PHYSICAL ENVIRONMENT. 201 



is responsible for providing the mechanism whereby root pressure is 

 set up. Anything that affects the protoplasmic mechanism will 

 affect this water mechanism. Extremes of temperature, either 

 heat or cold, will aboHsh it ; though, if the prejudicial effect has not 

 been carried on too far, recovery may ensue on a return to appropriate 

 temperatures. Anaesthetics and other poisons will also destroy it. 

 And we know something of the way in which these things work. 

 Normal protoplasm is a jelly-like substance, and possesses the pro- 

 perties of what physical chemists call colloids — that is, glue- or gum- 

 like substances. This means that the particles of which it is made 

 up are in a special state of aggregation. The molecules are large, 

 and the interspaces between them are occupied by water held there 

 in a peculiar condition and subject to certain laws which are now 

 gradually being discovered. This physical condition of water -\- the 

 molecules, forming the jelly-hke colloid, is, by virtue of its texture 

 and composition, the seat of peculiar forces which are only active so 

 long as the particular state of aggregation is maintained. Anything 

 that modifies this state immediately affects the nature and play of 

 the forces within it, and causes substances which can react with one 

 another to meet under different conditions. The results of such 

 reaction are thus seen to be different according to the manner in 

 which the particles of the viscous stuff are associated with each other. 

 And without going more fully into the matter we may say that one of 

 the predominating causes of the variety of chemical activities within 

 the protoplasm depends largely on the state of aggregation of the proto- 

 plasm itself, and the amount of water present is one of the factors that 

 affects this condition of aggregation. 



Many of the reactions are associated with so-called ferment actions. 

 Now no one has ever prepared a pure ferment, so we do not know 

 what any one of these numerous substances may really be like. It 

 is even possible that a large number of them may turn out not to be 

 separate substances at all in the ordinary meaning of the term, 

 but that they represent alternative structural conformations of certain 

 of the materials of which the cell stuff is composed. Their action is 

 usually simple, and often consists in merely incorporating or with- 

 drawing the elements of water in or from the fermentable substance. 

 It is true they are not all so simple, for others, and very important ones, 

 are concerned with promoting oxidation or some analogous change 

 in the substances on which they can operate. But the simpler 

 ferments — and they are the most numerous — only act as hasteners of 

 reactions which can be produced more slowly in a variety of ways. 

 iMoreover, and this is of fundamental importance, they are not them- 

 • selves consumed in the process. It appears then that their r61e 

 is a passive rather than an active one. They provide the form of 

 introduction to each other of the reacting bodies, e.g. water and the 

 fermentable material, and the result depends on the way these reacting 

 bodies are presented, and the conditions under which the introduction 

 is made. 



