4i6 BOTANICAL GAZETTE ' [may 



succulence in an organ. Briefly restated, whenever the water 

 content of a cell becomes low, some of the hexose-polysaccharides, 

 which have a low imbibition capacity, are converted into pentosans, 

 which have a high hydration capacity, the action having the force 

 of a regulatory adjustment, and as the change is irreversible, the 

 pentosans are accompanied by a permanent succulence, with all 

 of the implied alterations in metabohsm,^ including a very striking 

 change in the type of respiration, or of transformations in the 

 carbohydrates.^ 



It is notable that, while this change in the sugars takes place 

 in the cell, the type of transformations of energy changes completely, 

 but the approximate rate of respiration is not materially affected. 

 The nature and amount of the end products, however, may dift'er 

 materially from those of a respiration in thin leaves, notably in the 

 acid residues. It is in the mesh of reactions indicated that the 

 origin and the nature of succulence will be found, and whatever 

 causal value is attributed to the action of soil salts or of arid con- 

 ditions will rest upon their part in the conversion of the poly- 

 saccharides to pentosans. 



Acidity in succulents has been attributed by many writers, 

 including the authors of this article, to the imperfect oxidations 

 resulting from the lessened aeration of massive tissues, leaving a 

 residue of mahc acid, for example. Castilkja, however, presents 

 the example of highly acid thin leaves, which become succulent 

 under conditions similar to those which favor the transformation 

 of polysaccharide to pentoses in other plants. Instead of acidity 

 being a direct result of succulence, it is much more reasonable to 

 conclude that high acid residues may be characteristic of plants 

 which present a metabolic complex favorable to pentose formation 

 and to the development of succulence under certain environic 

 conditions. 



s Spoehr, H. a., The pentose sugars in plant metabolism. Plant World 20:365. 

 1918. 



'MacDougal, D. T., and Spoehr, H. A., The origination of xerophytism. 

 Plant World 2 1 : 245-249. 1918. 



Coastal Laboratory 

 Carmel, California 



