DEPARTMENT OF BOTANICAL RESEARCH. 91 



and in the table given that at 31° to 34° C. represents the largest 

 number of experiments averaged and consequently is the most illus- 

 trative. There is a constant rise in the rate of gas interchange with 

 the rise in the ratio, the difTerence between the highest and the lowest 

 being more than 300 per cent. The other groups are also in accord. 



With the increased evolution of carbon dioxide there is of course an 

 accompanying increase in the absorption of oxygen which, however, 

 must be less than the former, since the ratio is rising. 



In this very brief analysis of the series of experiments represented 

 by the accompanying table, some of the most striking points are 

 brought out, but the discussion is of course incomplete. It may be 

 safely said, however, that there can be no doubt of the association of 

 high acidities which are falling with high rate of gas interchange and 



CO 



with rising -^ ratios, while low acidities which are rising are accom- 



panied by low gas-interchange rate and lower ratios. Thus, as was 

 said in last j^ear's report, a young joint in which we may expect to 

 find the highest acidities will have the highest ratio and most rapid 

 intake of oxygen and output of carbon dioxide. Other things being 

 equal, a flaccid joint with low acidity will average the lowest ratio 

 and slowest evolution of carbon dioxide. It should be said, however, 

 that in certain instances the lowest ratio has been found with young 

 joints which are largely depleted of their acid. 



The gas samples from all of the experiments undertaken during the 

 summer of 1913 which touch upon the subject in hand have been 

 analyzed and the results calculated out by Miss M. E. Latham, and 

 all the other material which has accumulated since the writer began 

 this work in 1911 has been assembled. While it is evident that there 

 are several lines in which the problem must be investigated more fully 

 and that there are several new lines of work which show promise, it is 

 time that the mass of accumulated data be brought together in definite 

 form, and that has been the work of the present summer. 



Water-content and Acidity of Succulents, by E. R. Long. 



The investigations on the diurnal variation in the acidity of cacti 

 have been extended to the sahuaro (Carnegiea) and bisnaga {Echino- 

 cactus), with the additional object of discovering if the acidity of the 

 outer layer differs from that of the inner, and if the light and tempera- 

 ture effects upon the acid are different in the two regions. At the end 

 of a hot day as much as 15° C. difference may be observed between the 

 exterior and interior regions of a bisnaga 25 cm. in diameter. 



It developed that there was a decided difference between the acidity 

 values of the expressed juice from different regions of the same plant, 

 the acidity for a given tract in the cool of the early morning being 



