ACID ACCUMULATION IN SUCCULENTS 271 



surface, which in the sahuaro and bisnaga is approximately 5 

 mm. in thickness, so that the cells of the rest of the plant are 

 dependent for their carbohydrate nutriment upon diffusion and 

 conduction from the exterior. It seems logical to suppose, as 

 long as metabolism is continuous and no storing up takes place, 

 that the further an area is from its source of supply of sugars the 

 lower its concentration of sugar will be. And as the sugars are 

 continually being broken down with the production of acids the 

 concentration of the latter will be greatest where the sugar 

 concentration is greatest, that is, near the exterior of the plant. 

 It will be noticed in the tables that the situation is frequently 

 reversed at the close of the day, that is, that the acidity is great- 

 est in the interior and decreases toward the exterior, this being 

 due as explained above, to the daytime destruction of accumu- 

 lated acids. But it is significant that the nightly accumulation 

 of acid always leads to a higher acidity of the exterior than of 

 the interior. 



SUMMARY 



The acidity of the sap of Echinocactus wislizeni and Carnegiea 

 gigantea is higher in the early morning than at sunset, the acids 

 formed in the metabolism of carbohydrates tending to accumu- 

 late during the night and being partially destroyed by higher 

 temperature and photolysis in the succeeding day. The acidity 

 of the sap of these species following the nightly accumulation of 

 acid is higher in the outer than in the inner regions. This higher 

 acidity is probably resultant from a higher concentration of sugar 

 in the outer parts, that is, in the region of sugar formation. The 

 diurnal difference in acidity of the sap becomes smaller in succes- 

 sive zones as the center of the plant is approached, being as much 

 as 50% in the exterior and slight in the innermost tissues. In 

 the outer parts where carbohydrates are plentiful, sugar metabo- 

 lism is great and acid production and nightly acid accumula- 

 tion are large ; these parts are on the other hand the most exposed 

 to the action of light and heat, so that the daily destruction of 

 acid is great. Accordingly here diurnal differences in acidity 

 are large. In the innermost tracts of tissue carbohydrates are 



