28 



ACIDITY AND GAS INTERCHANGE IN CACTI. 



Returning to the young joints, it is not remarkable that they should show a 

 higher average acidity than the mature tissue, for, as will be seen later, their 

 oxidation processes must be going on at a more rapid rate, as also is the photo- 

 synthesis, which must in the last analysis afford the material from which the 

 acid is formed. In the same way, the greater fall of acidity in young tissue 

 is also probably due to the more active oxidation. It is not at all impossible, 

 also, that the acid destruction may be favored by the more delicate nature of 

 the young epidermis, which would allow of greater penetration of the sun's 

 rays and thus bring about more rapid deacidification. The elongation of the 

 young joints after the bud begins to develop is exceedingly rapid if the condi- 

 tions be suitable, but after the first season those of Opuntia versicolor at least 

 do not grow greatly in length. 



3 AM 7 II 



0.70A 



\ 



V 



20'C 



\ 



50A 



16 C C 





30A 



FIG. 2. Graph showing acidity of Opuntia versicolor in terms of c.c. of decinormal 

 alkali per gram fresh weight. 



= acidity. = temperature. 



The ordinates read for either acidity or temperature. Thus 0.30A signifies an acidity of that 

 amount or also stands for 10 C. temperature, similarly 12 C. may read as well as 0.35 acidity. 

 The'plants were in the open and uncovered. Average of experiments in table 8. 



12 



m'dt 



456 8 10 noonpWz 4 5 



\ 





FIG. 3. Graph showing acidity of Opuntia versicolor in terms of c.c. of decinormal alkali per 

 gram fresh weight, in comparison with diurnal change of temperature of air and of 

 tissues within the plant. 



= acidity. =air temperature. 



= temperature within the plant. 



The ordinates read for either acidity or for temperature. Plants in the open, uncovered. 

 Average from experiments in table 9. 



From the accompanying graphs (figs. 2 and 3), which have been made from 

 the experiments set forth in tables 8 and 9, it may be seen that the diurnal 

 course of acidity runs almost opposite to that of temperature, from which, 

 however, it should not be assumed that temperature is necessarily the only 



