272 ESMOND R. LONG 



less plentiful, acid production is correspondingly less, and the 

 increasing insulation as the center is approached tends to mini- 

 mize diurnal temperature and light differences, so that in these 

 parts diurnal differences in acidity are small. The movement of 

 sap of low acidity from the central cylinder to the peripheral 

 layers of these thick succulents has little effect upon the acid con- 

 tent of these tracts, while variations of great amplitude result 

 from the conjoint action of light and temperature. 



