4o6 BOTANICAL GAZETTE [may 



and another with fleshy leaves growing on the sandy foreshores 

 under arid, but not sahne, soil conditions. The succulent leaves 

 owe their increased thickness to the enlargement of elongated cells 

 vertical to the surface of the leaves. 



2. The thin leaves show an acidity double that of the fleshy 

 t>pe, and have a relatively greater dry weight. 



3. The fleshy leaves, fresh and in a dried condition, present 

 swelling reactions similar to those of sections of the joints of platy- 

 opuntias, indicative of cells high in pentosans, or mucilages. The 

 behavior of these organs is different in many important particulars 

 from that of thin leaves, which swell more in acid than in alkaline 

 solutions, the reverse taking place in succulent leaves, in parallelism 

 to Opuntia. 



4. Differences in the swelling reactions of dried leaves of both 

 kinds are to be ascribed to the adsorption of the contained acids 

 and salts of different amounts in the two cases on cell colloids, high 

 in pentosans in one case and hence presenting characteristic 

 coagulatory effects. 



5. It has been established by researches not described in this 

 paper that the reduction of the water content of the cell below a 

 certain point results in the conversion of polysaccharides, which do 

 not show a high imbibition capacity, to pentosans, which mixed 

 with nitrogenous substances have an enormous hydration capacity. 



6. Succulence, therefore, may originate as it is seen to occur in 

 Castilleja as a direct result of aridity. Species of Ericameria and 

 Erigeron with a distribution similar to Castilleja display thin and 

 succulent leaves corresponding in the same manner to the 

 environment. 



7. High acidity may not be taken as a result of succulence. It 

 is probably more nearly correct to assume that succulence may 

 develop only in plants which have a carbohydrate metabolism 

 characterized by large acid residues. 



The bulk and durability of succulents have made them readily 

 available for chemical studies, and these features are responsible 

 in part for the fact that carbohydrate metabolism and respiration, 

 photosynthesis, the formation and fate of acids, the oxygen-carbon- 

 dioxide ratio, and other features have all received contributions 



