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



such use of its carbohydrates is of course accompanied by a 

 limited growth, particularly in branches and leaves where the 

 effects of aridity would be greatest. 



The separate types of transformation of carbohydrates might 

 take place in the same plant, in different cells. Thus some of 

 the massive cacti have shoots from which the power of branch- 

 ing has been entirely lost and the stems are reduced to short, 

 cylindrical, swollen or globose forms. The external layers of 

 such plants exhibit the typical xerophytic anhydrous wall-forma- 

 tions while the cortical elements have been the scene of transfor- 

 mations of sugars resulting in succulency. 



It has been suggested above that succulence based on the 

 conversion of polysaccharids into pentosans may occur in plants 

 with an incomplete type of respiration which is denoted by 

 large proportions of residual acids. The probability of this 

 suggestion rests upon a wealth of determinations of the acidity 

 and sugar content of the cacti and of Mesembryanthemum. Simi- 

 lar full and detailed analyses of xerophytes are not yet available. 



An examination of the metabolism of these plants would in 

 all probability reveal physical conditions which facilitate or lead 

 to exaggeration or acceleration of the formation of anhydrous 

 wall material. 



The exposure of a plant to arid conditions might be expected 

 therefore to be followed by a retarded development due to the 

 lack of water necessary for the hydration of cell colloids in 

 growth, by the accelerated formation of pentosans or mucilagi- 

 nous material in the cells, leading to hypertrophy of the paren- 

 chymatous elements, or by the increased formation of wall 

 material, especially in the external layers constituting the es- 

 sential feature of xerophytism. One or all of these responses 

 might be exhibited by plants under experimental conditions, and 

 the subject is one which deserves the renewed attention of the 

 experimentalist.'^ The results now available consist chiefly of 

 data for anatomical comparisons. 



* Eberhardt, Ph. Influence de I'air sec et de I'air humide sur la forme et sur 

 la structure des vegetaux. Ann. Sci. Nat. VIII. 18:61-153. pi. 1. Jan. 1903. 



