50 BIOLOGY: GENERAL AND MEDICAL 



confused with what are called instincts, at the founda- 

 tion of which they undoubtedly lie, that they are apt to 

 be lost sight of. 



The struggles of a starving man to secure food by 

 honest employment, by change of locality, by solicita- 

 tion or by theft, though rarely so regarded, are as 

 certainly determined by positive sitotropic internal 

 chemical conditions and by the necessity for the 

 reintegration experienced by his cells and expressed as 

 hunger, as is the more simple behavior of the hydra or 

 the amceba. 



B. Hydrotropism, or response to the stimulating influ- 

 ence of water, is an important form of chemotropic re- 

 action the effects of which are observable among nearly 

 all forms of life. Care must be taken, however, to 

 separate such activity or quiescence as may depend 

 upon the presence or absence of water with favorable 

 and unfavorable conditions depending thereupon, from 

 the real hydrotropic reactions in which the active organ- 

 ism behaves peculiarly in its efforts to effect the best 

 utilization of available water. 



As usual the reactions consist of positive and negative 

 movements. 



The myxomycetes show distinct positive hydrotropic 

 reactions. Thus, if one be placed upon a piece of blotting 

 paper so arranged as to be dry at one end and damp at 

 the other, the amoeboid movements of the plant slowly 

 bring it to the moist end of the paper. If, now, the 

 paper have its position reversed, so that what was 

 formerly the dry end becomes the moist end and vice 

 versa, the organism gradually spreads its amoeboid net- 

 work more and more toward the moisture until, in the 

 course of time, it has again traversed the length of the 

 paper. This is positive hydrotropism and represents the 

 common form. 



Seedling plants usually arrange themselves in such 

 manner that the stems grow upward toward the light 

 and heat, while the roots grow downward into the soil 



