Fluctuating or Alternating Hydration Effects. 91 



not at all or very slowly. The swelling of the succulent leaves is 

 therefore high after extraction by reason of the presence of these sub- 

 stances, which have been partially freed from the retarding action of 

 the organic acids and salts. 1 



The conversion of the diffusible sugars to the mucilaginous pentosans 

 is one of the alterations which may result in the cell as a result of partial 

 desiccation, with a striking morphogenetic result as indicated. Under 

 certain conditions of desiccation, the lessening of the water-content of 

 the cell accelerates the formation of the anhydrides of which wall 

 material is composed, with the result that the hydration capacity of 

 the product in this case is less than that of the polysaccharids, and in 

 addition to this direct diminution of the water-absorbing material of 

 the cell, the production of the heavy wall at an early stage of the 

 development of the cell checks growth, resulting in the restricted 

 shoots or organs with indurated membranes which are characteristic 

 of xerophytic plants. 2 Two striking and highly important vegetational 

 types are thus seen to result more or less directly from the action of the 

 environment upon the cell-colloids, hi accordance with a view expressed 

 by the author in a previous publication. 



1 MacDougal, Richards, and Spoehr. The basis of succulence in plants. Bot. Gaz., 67: 405 

 1919. 



2 MacDougal, D. T. The Salton Sea, etc. Carnegie Inst. Wash. Pub. No. 193. 1914. See 

 p. 179. Also MacDougal and Spoehr. The origination of xerophytism. Plant World, 21: 245. 

 1918. 



