ORIGINATION OF XEROPHYTISM 247 



These 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 Labora- 

 tory. Determinations 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 vari- 

 ous transformations is a change of polysaccharids into pento- 

 sans 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 increase of the cells, resuKs in succulence. Not all plants in 

 which such transformations take place become succulents, but 

 two species have been observed in which individuals growing 

 under arid conditions become succulent and those elsewhere 

 maintain their mesophytic character. One, Castilleja latifolia, 

 was found to be characterized by a high acidity of the sap in the 

 thin leaves, and a lower acidity in the succulent individuals. It 

 is suggested that plants which have a type of respiration result- 

 ing in a large proportion of residual acids may be capable of 

 succulency, but this is a matter which has not yet been substan- 

 tiated by any facts beyond those cited. ^ 



The depletion of the water supply may, under circumstances 

 as noted above, result in the conversion of polysaccharids into 

 pentosans which take up and hold in a mucilage large proportions 

 of water. This of course is but one of the possibilities. Under 

 other conditions a low water content causes the formation of the 

 anhydrides, of which wall-substance or cellulose is an example, or 

 more properly speaking such action is increased or accelerated, 

 and the plant structure thus becomes hard and indurated, and 



* Schloesing, Th. Vegetation comparee du tabac sous glocke et h, I'air libre. 

 Compt. Rend. Paris 69: 353. 1869. 



5 MacDougal, Richards and Spoehr. The basis of succulence. Bot. Gaz. 

 In press. 



Spoehr, H. A. The pentose sugars in plant metabolism. Plant World 12: 

 365. 1917. 



