246 D. T. macdougal and h. a. spoehr 



effects of aridity for the induction of both succulence and 

 xerophytism. 



Thus it was said in 1914, 



A general parallel is offered by the behavior of a dish of hydrated 

 gelatine which may be exposed to the air under high evaporative condi- 

 tions. If the conditions of desiccation were of the intensity of the 

 desert, the outer surface would soon become coagulated and the hard- 

 ened surface would thus effectually check the loss of water from the 

 layers beneath. This in the final analysis is what takes place in the 

 body of the plant and the coincidences are so sweep- 

 ing and universal as to suggest that a causal connection is present.^ 



This general idea however had but little weight, as it did not 

 include any explanation of the mechanism of the implied changes. 

 It would seem to require no argument to establish the statement 

 that any form or condition of the plant dependent upon its water- 

 relations would be directly connected with the imbibitional or 

 water-holding capacity of the cells. It has long been recog- 

 nized that the structure and chemical composition of a plant 

 may be modified by its water relations during growth, but until 

 recently available analyses did not include data upon which 

 any explanation of the present problem might be based. This 

 was finally found in the carbohydrate metabolism, and in the 

 imbibitional action of the mucilages and proteins in plant cells. 



If we turn to the previously available analyses of plants and 

 examine them for the purpose of determining changes in the 

 carbohydrate content as influenced by humidity and aritity, 

 some facts of great interest are encountered. The representative 

 data of Schloesing show the following features of tobacco plants 

 grown in dry and moist atmospheres. 



3 MacDougal, D. T. The Salton Sea. Publ. Carnegie Inst, of Wash. No. 

 193. 1914. p. 178. 



