52 BIOLOGY: GENERAL AND MEDICAL 



it usually results in the formation of spores, or the 

 entrance of the organism upon an encysted or latent 

 stage. Among the higher plants inability to assume 

 such forms and to transport themselves to a new neigh- 

 borhood result in death; in the higher animals it results 

 in various movements for the purpose of obtaining the 

 essential moisture. Thus land crabs are compelled to 

 travel every day or two to the water for the purpose of 

 moistening the branchiae, and among still higher animals 

 the drying of the pools and springs leads certain of the 

 fishes and amphibia to bury themselves deeply in the 

 mud, where they remain inactive until the return of 

 rain. Still higher animals such as the mammalia may 

 be compelled to make long periodical migrations corre- 

 sponding to the periods of rainfall and drought. Fail- 

 ure of the water supply is followed by death in such 

 cases. 



C. Oxytropism, or Response to the Stimulating Effects 

 of Oxygen. 



This form of chemotropism results from the necessity 

 which all living things experience with reference to 

 oxygen, which is essential to all forms of life. For 

 most living things the free oxygen of the atmosphere is 

 sufficient, but a few forms of life are unable to make 

 use of it and are obliged to secure such oxygen as they 

 need by the analysis of compounds containing it. This 

 is best exemplified by the anaerobic bacteria, which, 

 appearing to be overstimulated by the uncombined 

 element, show no signs of activity until it is completely 

 excluded, when they begin to analyze the compounds 

 from which they may obtain it. The greater the avail- 

 able oxygen in these compounds, the better and more 

 actively the organisms multiply, so that solutions of 

 carbohydrates form the best substratum for their 

 cultivation. 



When, on the other hand, aerobic bacteria occur under 

 conditions which make it difficult to secure sufficient un- 

 eombined oxygen for their purposes, interesting phenom- 



