THE FORMS OF PHYSICAL ENERGY USED BY PLANTS 403 



re-expand when supplied with it. The distinction is really one of little 

 value, since in both cases the responding mechanism is a product of vital 

 activity, and physical responses of this character are often capable of 

 frequent repetition and may take place against considerable resistance *. 



The osmotic energy of the cell bears no definite or constant relation- 

 ship to the energy consumed in the production and accumulation of the 

 osmotic materials 2 . The former is entirely dependent upon the number of 

 molecules, and remains the same whether energy is absorbed or liberated 

 during their production. Furthermore, an osmotic substance may be directly 

 absorbed from without and accumulated in the cell by passive secretion, but 

 it exercises precisely the same osmotic action as if it were a product of 

 anabolic metabolism or of katabolic respiration. The hydrolysis of insoluble 

 starch by an enzyme produces osmotically active sugars, and the osmotic 

 action is doubled when the large molecule of cane-sugar is converted by 

 invertase or by dilute sulphuric acid into two molecules of grape-sugar. 

 Similarly the reverse process, or the conversion of a soluble into an 

 insoluble substance, will lower the osmotic pressure. 



As in the case of a compressed gas, the presence of a dissolved substance 

 in a cell only enables a limited amount of external work to be done, for 

 with the increase in volume of the growing cell, the solution is diluted 

 and the number of molecules per unit volume decreased, so that here, as in 

 the case of an expanding gas, the pressure falls 3 . Hence the maintenance of 

 growth involves a continued production of osmotic materials. 



When a gas does work in expanding its temperature falls, and in exactly 

 the same way when work is done by osmotic energy, as is the case when a 

 cell grows by plastic stretching, the osmotic pressure falls. In both cases 

 the work done is due to the energy of the moving molecules, and except in 

 so far as the temperature affects the velocity of the molecules, and hence 

 also the osmotic pressure they exert, it is immaterial to the plant whether 

 its temperature is kept higher by respiration than that of the surrounding 

 medium or whether it is kept permanently lower by transpiration. During 

 transpiration itself the heat absorbed from without does work in altering the 

 water from the liquid to the gaseous state, and this work is externally 

 manifested when water is raised up a vertical stem by the suction of the 

 leaves. 



Apart from its chemical quality the value of a substance as a source of 



1 Pfeffer, Studien zur Energetik, 1892, p. 236. 2 Id., pp. 170, 172, 196, &c. 



3 On the work done during the expansion of gases see the textbooks of Physics. Rodevvald 

 (Ber. d. hot. Ges., 1892, p. 83) erroneously assumes that the mechanical equivalent of the heat of 

 combustion of a substance must always be greater than its power of doing work by its osmotic 

 action, and that bodies not produced in the cell can do no osmotic work. The latter statement 

 hardly coincides with the fact that a passive or active absorption from without and an accumulation 

 in the cell of soluble substances is possible. 



D d 2 



