262 ESMOND R. LONG 



ferent regions of the same plant. A difference of 15°C. was noted 

 in a bisnaga 30 cm. in diameter between the temperature of the 

 tissue just under the epidermis and that of the tissue in the center 

 of the plant at 5 p.m. after a warm day. This is approximately 

 equal to half of the diurnal variation in the temperature of the 

 air at the season in which these experiments were conducted. 



The material for this investigation was all obtained from the 

 eastern slope of the Tucson Mountains within a radius of three 

 or four miles from the Desert Laboratory. The method adopted 

 in the analysis was as follows: Only as many plants as could be 

 studied in three or four days were secured at one time. These 

 were exposed one at a time to the sunlight throughout the 

 day and split longitudinally late in the afternoon, the half which 

 had been most exposed being taken for analysis immediately, 

 the other being laid aside for analysis in the early morning. 

 The latter portion was laid cut surface down on a flat table so 

 that as little evaporation as possible might take place from the 

 injured surface, and in the morning the further precaution was 

 taken of cutting away about an inch of this surface layer, which 

 would be as deep as any abnormal evaporative or oxidative 

 action of the air might have occurred. The actual analysis was 

 made in this manner: Sample portions were taken at a uniform 

 distance from the base, 4 from each split plant and numbered as 

 indicated in figure 1. A and B were close together and taken 

 within the ridges of the plants, A just inside of the chlorophyll 

 bearing tissue, B a little deeper. A and B were thus from a re- 

 gion much more exposed to light and heat than the other samples 

 and A in turn more than B. These relations are indicated in 

 the diagrams of the plants cut in cross section. The samples thus 

 obtained were pressed out in a fruit press and the products 

 warmed rapidly and titrated against g> NaOH with phenol- 

 phthalein as indicator. In the tables to follow, the acidity of the 

 sap, which was determined by the titration of 25 cc. samples, is 



4 MacDougal and Spalding, Carnegie Inst., Wash., (Publ. 141) have shown that 

 the proportion of sap to dry tissue varies considerably within the same plant in 

 the sahuaro and bisnaga, the water content being greatest near the apex of the 

 trunk and least at the base. 



