DEPARTMENT OF BOTANICAL RESEARCH. 99 



ture at depths of 10, 50, and 100 cm., and of evaporation covering a 

 period of 45 days in July and August, and soil samples were collected 

 for mechanical analysis and other determinations. Anatomical and 

 other studies wdll be carried on during the winter months in the labora- 

 tory and greenhouse at the University of Minnesota. 



The Origination of Xerophytism in Plants, hy D. T. MacDougal and 



H. A. Spoehr. 



The part played by aridity as an evolutionary factor in the deriva- 

 tion of land forms is v/ell recognized by students of phylogeny. It is 

 well known that only plants showing speciaUzed habits coupled with 

 well-defined anatomical features may continue to exist in places having 

 pronounced desert conditions. The idea that aridity stands in a causal 

 relation to the characters of desert vegetation bulks large in determin- 

 ing our thinking of these forms. It is to be seen, however, that the 

 matter has been dealt with hitherto as if the effect of aridity did not 

 make an impression upon the plant, its Hving matter, or accessory 

 structures directly. The combined effects of rapid evaporation and 

 undeveloped drainage in desert regions have resulted in accumulation 

 of a greater proportion of salts in the soils than in well-watered and 

 freely drained soils, and the strand habitats of the sea-shore succulents 

 are also high in salts, and this has caused attention to be directed to the 

 possible effects of these substances in inducing the succulence of plants 

 both on strands and in the desert. The formation of the spiny plants 

 of the desert was also attributed to the possible osmotic action of these 

 substances. 



It has long been recognized that the structure and chemical com- 

 position 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 imbibi- 

 tional 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 aridity, it will be seen that the 

 figures show a greater amount of cellulose and a lesser amount of starch 

 in the plants developed in the drier atmosphere. 



The possible significance of the transformations in question was not 

 realized, however, until a long series of detailed analyses of the sugar- 

 content of the cacti was made at the Desert Laboratory. Determina- 

 tions were made in all stages of development of the plant, in all the 

 seasons, and of material subjected to various experimental conditions. 

 Prominent among the various transformations is a change of poly- 

 saccharids into pentosans or mucilages, a conversion of carbohydrates 

 of but little hydration capacity into others which have a large coefficient 

 of imbibition. This change, when accompanied or followed by the 



