THE FORMS OF PHYSICAL ENERGY USED BY PLANTS 405 



Imbibition and surface-tension energy are probably of as great im- 

 portance in vital economy as osmotic energy, and all these forms of energy, 

 but especially the first named, may produce pronounced external mani- 

 festations, as during the swelling of wood, of seeds, or of starch, or during 

 the imbibition movements of the awns of certain seeds which are repeated 

 with each drying and moistening. The energy manifested in movements 

 of this kind naturally bears no relation to the consumption of energy 

 involved in the production of the reacting mechanism, since the latter 

 merely directs the operation of the heat-energy derived from without. The 

 same applies to movements due to changes of surface-tension resulting from 

 diffusion, evaporation, or the action of electrical currents. 



SECTION 89. Chemical Energy. 



The production and accumulation of various substances enables the 

 plant to utilize osmotic and surface-tension forces, and metabolism may 

 also produce electrical currents, though it is uncertain whether these are 

 of much value in the vital economy. By means of the former forces, 

 however, the plant is able to convert heat into work, whereas the direct pro- 

 duction of heat by respiration serves no such useful purpose as does the 

 fire in a steam-engine, and it is almost entirely dissipated by radiation, 

 conduction, and evaporation. In other words, the protoplast is neither 

 a thermodynamic nor an electro-dynamic machine. 



It is by no means certain to what extent chemical energy may be 

 directly utilized for chemical purposes, either within the plant or outside 

 of it. In any case, mechanical work is done when a chemical action 

 involves the dissociation and recombination or rearrangement of molecules. 

 The same applies whenever a chemical action involves an increase in 

 volume which takes place against the atmospheric pressure, or when 

 a substance is crystallized or precipitated in a colloid medium whose 

 resistance has to be overcome. In the latter case, however, if the pro- 

 duction and separation of the substance are distinct phenomena, the 

 separation may be regarded as a physical manifestation of volume energy 

 independently of whether it is produced by crystallization, by absorption, 

 or by the removal of a solvent. 



Phenomena of this kind play a prominent part in all vital actions, for 

 the growth of the protoplasm by intussusception involves the intercalation 

 of new particles between pre-existent ones. Chemical changes, surface- 

 tension energy, the chemical affinities of the various materials, and imbibi- 

 tion may all take part in this process and determine whether the new material 

 shall be tacked on to particular micellae or placed between them. During the 

 growth by intussusception of the cell-wall, the influences radiating from the 

 pre-existent particles take an important part in determining the character 



