ACIDITY DETERMINATIONS. 



35 



decrease was maintained up to 60 hours, after which a slight rise was to be 

 noticed. This last-named circumstance may very well be due to the fact that 

 after 1\ days the wound periderm had formed sufficiently to protect the cut 

 surfaces. The subsequent experiments with wound respiration bear out this 

 probability. The acidity of a parallel specimen, which had not been wounded, 

 shows the ordinary slight decrease in acidity which comes as a result of long- 

 continued darkness. It may be inferred that this diminution of acid is largely 

 the effect of the increased exposure to the oxygen of the air consequent on 

 the injury. 



MISCELLANEOUS. 



In tests made to determine the region of highest acidity it was found that 

 the cortex is usually more acid than the pith, while leaves are of about the same 

 acidity as the outer layers. Young fruits did not show the increased amount 

 of acid that might be expected, though they do perhaps contain somewhat 

 more acid than the ordinary tissues. It did not appear that the acidity varied 

 to any noticeable extent in different parts of any one joint. A long, young 

 joint showed at the base a similar acidity to that at the tip. Similarly, none 

 of the mature joints that were tested showed any indication of inequalities in 

 the longitudinal distribution of the acids. 



3AM. 



9P.M. I2m'd't. 3AM. 6AM 9A.M. iSnoort 



FIG. 4. Graph showing acidity of Mammillaria grahami in comparison with diurnal temperature 



curve. The acidity is given in terms of c.c. of decinormal alkali per gram fresh weight. 



= acidity. = temperature. 



Attention has already been called in the chapter on experimental methods to 

 the differences of acidity in shoots of unequal ages. The young growing joints 

 are distinctly more acid, on the average, than the mature ones, both as to the 

 total amount present and as to the concentration of the juice. Shoots of the 

 age of 1 or 2 years are hardly separable on the basis of their acidity, but at 3 

 years there is some decline in total acidity (table 22). This may very well be 

 due to the increase of woody tissue, which would greatly increase the dry 

 weight and hence diminish the total aciditj' per gram. The acidity of the 

 pure juice is practically identical at 1, 2, and 3 years, which would indicate 

 that the activity of the living tissue is of the same magnitude in all three years. 

 In the two instances where shoots bearing flower-buds were used the acidity 

 was found to be considerably higher, showing that in this condition the tissues 

 are probably more active. The investigation made to ascertain possible locali- 

 zation of acidity was not at all complete. The tests were more for the purpose 

 of deciding what precautions should be observed in the selection of material 



