266 ESMOND R. LONG 



in morning acidity over preceding afternoon acidity, is greater 

 in the case of B than in A. It is probable that the lateness of 

 the hour of analysis (9 a.m.) is partly responsible for this condi- 

 tion, i.e., that the photolysis of acid in A was already under way. 

 From B to D the usual condition of a progressive decrease in the 

 subtraction value, is observed, until at D the acidity of morning 

 and afternoon sap is practically the same. Bisnaga No. 27 pre- 

 sents a contrast to all the other specimens studied in that the 

 sap of A is more acid in the afternoon than in the early morning, 

 and the subtraction value becomes larger instead of smaller as 

 the center is approached. No satisfactory explanation can be 

 given for this discrepancy. Apparently metabolism was not 

 very active, for the differences between day and night acidity 

 are not great at any point. Owing to adverse weather conditions 

 early morning analyses were not made of Nos. 30 and 31, these 

 being included in the protocols merely to show some other rela- 

 tions of interest in the general discussion. 



DISCUSSION 



A glance at the tables shows a constant feature with regard to 

 the water distribution. In morphologically similar tissue, that 

 is from A to D inclusive in the bisnaga and in A and B in the 

 sahuaro, the dry weight of the tissue decreases as the center is 

 approached. (E of the bisnaga and C of the sahuaro, each within 

 the fibro-vascular cylinder of the plant are more woody than the 

 tissues peripheral to the ring and are characterized by a relatively 

 low water content and low acidity.) This is presumably the re- 

 sult of an active transpiration in which water is removed from 

 the superficial tissues more rapidly than it is supplied by conduc- 

 tion and diffusion from the interior. It would be expected that 

 in the nightly checking of transpiration the outer tissues would 

 tend to " catch up" in water content at the expense of the inner 

 ones, that is, that the inside tissues would become drier and the 

 outside ones wetter during the night, and that the morning anal- 

 ysis would show a more even distribution of the water than in 

 the preceding evening. And this is actually found to be the case 



