480 GENERAL CONSIDERATIONS [Ch. XIX 



Let us now consider the mechanics of the acceleration of 

 growth by various agents. The mechanics of the effect of 

 food on growth varies of course with the food ; some supply- 

 ing the energy or the material for assimilation, other kinds 

 furthering secretion, and still others going to build up the 

 molecules which do the work of vital imbibition. Whether 

 the poisons which, like zinc sulphate, stimulate growth affect 

 chiefly the assimilatory or the imbibitory process, is unknown; 

 but the slowness of the result suggests a modification of assimi- 

 lation. The effects of water and of solutions seem to be chiefly 

 upon imbibition; that of deformation and wounding chiefly 

 upon assimilation ; that of electricity is uncertain ; that of 

 light is probably chiefly upon assimilation; and that of heat 

 upon imbibition. These conclusions are, however, tentative; 

 experiments are needed to test them further : this is one of the 

 next tasks in the investigation of growth. 



2. Modification of Direction of Growth — Tropism. — The 

 tropic effect of an external agent occurs only when the agent 

 does not act uniformly from all sides upon the growing organ- 

 ism, but constantly from one side. Hence this effect will be 

 marked only in organisms which for a considerable period 

 present the same surface to the action of the agent. Such an 

 effect is characteristic therefore of sessile plants and animals. 



The turning of an organ with reference to an external agent 

 may be either a gross effect or a specific response effect of the 

 agent. As examples of the gross effect may be cited the cases 

 of false traumatropism and false electrotropism, in which the 

 turning is due to the death or injury of the tissue on the con- 

 cave side of the organ. All cases of true tropism are cases of 

 response to stimuli : such are, chemotropism, hydrotropism, 

 thigmotropism, traumatropism, rheotropism, geotropism, elec- 

 trotropism, phototropism, and thermotropism. 



The mechanics of these tropic responses may now be briefly 

 considered. In general the tropisms are growth phenomena, 

 for they are due to enlargement of the bending organ on one 

 side. The growth seems to be due either to assimilation or 

 imbibition, or both, secretion playing no part. It is not easy 

 to say whether in any case assimilation or imbibition is the 

 more important. An inference can be drawn, however, from 



