56 The Mechanistic Conception of Life 



can act, much as acids do for the heUotropism of certain 

 animals, namely, to increase the sensitiveness to certain stimuli, 

 and thus can lead to tropism-like movements or actions directed 

 toward a goal. 



IX 

 Besides light and the electric current, the force of gravity 

 also has an orienting influence upon a number of animals. 

 The majority of such animals are forced to turn their heads 

 away from the center of the earth and to creep upward. It was 

 uncertain for a long time how the orientation of cells in relation 

 to the center of gravity of the earth could influence the rate of 

 the chemical reactions within, but it has been suggested that 

 an enlargement or shifting of the reacting surfaces formed the 

 essential connecting link. If it is assumed that in such geo- 

 tropically sensitive cells two phases (for instance, two fluid 

 substances which are not at all, or not easily, miscible, or one 

 solid and one fluid substance) of different specific gravities are 

 present, which react upon one another, a reaction takes place 

 at the surfaces of contact. Every enlargement of the latter in- 

 creases the mass of reacting molecules. A shifting of the surfaces 

 would act in the same manner. Finally, a third possibility re- 

 mains which could perhaps be realized in plant roots and stems. 

 If in the geotropically sensitive elements two masses of differ- 

 ent specific gravity are present, only one of which reacts to the 

 flowing sap in the center or the periphery of the stem, the cells 

 of the upper side of a stem which is laid horizontally will ac- 

 quire a different rate of reaction from those of the lower side, 

 because in the former the specifically heavier substances are 

 directed toward the center of the stem, while in the latter the 

 specifically lighter ones are directed toward the center. Con- 

 sequently, one side will grow faster than the other, hence the 

 geotropic bending.' In the frog's egg, we can actually demon- 

 strate directly the existence of two substances of different 



1 Chapter on "Tropisms" in Dynamics of Living Matter. 



