404 SOURCES AND TRANSFORMATIONS OF PLANT-ENERGY 



energy is not measured solely by its heat of combustion, nor is the series 

 of transformations it may undergo in the service of the organism immaterial. 

 The osmotic action depends solely upon the number of the molecules and 

 their kinetic energy, and not upon their potential chemical energy as 

 measured by their heat of combustion. Hence substances may exert a 

 powerful osmotic action when in solution, although completely oxidized 

 compounds. A substance which first exercises an osmotic function and is 

 consumed at a later date in respiration is more important physiologically 

 than one utilized for one function only. If the product of oxidation is to 

 retain an osmotic function and yet yield energy during its production, it is 

 far better when substances like organic acids, having a low heat of com- 

 bustion, are produced by respiration in place of the volatile carbon dioxide. 

 Thus when a molecule of glucose is oxidized to three molecules of oxalic 

 acid not only is the osmotic action trebled but also the greater part of the 

 available chemical energy is set free in the form of heat 1 . 



SURFACE-TENSION determines the shape of drops of liquid, but it is 

 not yet certain to what degree amoeboid movements are the results of 

 spontaneous and induced changes of surface-tension coupled with alterations 

 in the cohesion of the outer layers. The same applies to pulsating vacuoles, 

 while protoplasmic streaming has been suggested to be due to the pro- 

 duction of differences of surface-tension in the regularly arranged bipolar 

 particles of the protoplasmic emulsion by the action of inwardly- or 

 outwardly-directed electrical currents. It is, however, uncertain how far 

 autogenic changes of surface-tension are responsible for the changes of 

 shape of the nucleus, of plastids, or of the reproductive cells of flowering 

 plants. Surface-tension may also take part in determining the fusion or 

 non-fusion of gametes, and the movements of cilia 2 . 



When a solid is finely divided the surface-tension of its component 

 particles becomes of increasing importance, since the inwardly-directed 

 pressure exerted by it on a spherical particle is inversely proportional to 

 the radius of the particle. The force with which particles of water or of 

 other fluids are able to penetrate between the molecules or micellae of 

 substances capable of imbibition and of swelling is the result of molecular 

 forces akin to that of surface-tension. Absorption phenomena of this kind 

 form a part of physical chemistry, and indeed the absorption of certain 

 substances involves a loose chemical union, so that the process may be 

 regarded as a physical or as a chemical one according to the point of view. 

 Furthermore, many kinds of imbibition are produced in much the same 

 way as the so-called solid solutions, as when two metals are placed in 

 contact and the particles of one penetrate the other. 



1 Pfeffer, 1. c., pp. 173, 197 ; Rodewald, 1. c. 



2 For theories of streaming cf. Ewart, Protoplasmic Streaming in Plants, 1903, p. 108. 



