300 BOTANICAL GAZETTE [APRIL 
discernible, but in most samples the acidity is less in the rotted 
than in the sound samples. In a majority of cases the fresh 
material shows a higher acidity than the material after storage 
in the laboratory, but the data are not conclusive on this point. 
When the analyses are arranged as in fig. 5, it is seen that in the 
fresh samples the resistant varieties have a somewhat higher 
acidity that the non-resistant, but such differences do not obtain 
in the stored and in the rotted samples. It is also evident that the 
acidity in plums neither increases nor decreases to any appreciable 
degree as ripening progresses. This is in accordance with most of 
the observations on other fruits. 
Since the differences in acidity between resistant and non- 
resistant varieties are not great, and since the growth of the fungus 
in the tissue tends to lower the acidity to only a slight extent, there 
is not much evidence that unfavorable acidity is an important 
factor in resistance of plums to brown rot. 
TITRATABLE AcipiITy.—In figs. 3 and 5 the determinations of 
the titre of the juices follow the trend of the P, values in reverse 
order, that is, when the titre is high, the hydrogen-ion concentration 
is high. ‘There are some exceptions to this, which may be due to 
differences in buffers in the juices. In fact, in the fresh samples 
the resistant varieties have both a lower P, and a lower titre, which 
would indicate that the acids are much more highly dissociated. 
The data for oxalic acid show that these varieties do have slightly 
more of it than the others, but the amounts involved are too small 
to amount to the differences in H-ion concentration. The character 
of the buffers may determine this. A careful study of the graphs 
shows that the changes in titratable acidity are relatively greater 
than the corresponding changes in H-ion concentration. This 
becomes more evident when numerical values are used for the 
comparison. The average percentage decrease in titre from 0 to 
¢ in all the samples is 17. The average percentage increase in Pu 
values is 9 (assuming a theoretically possible increase to Ps=7)- 
This would indicate a consumption of acid by the fungus, rather 
than a production of buffer, in modifying the reaction during 
rotting. It was hoped that the malic acid determinations would 
give direct evidence on this point. These determinations will be 
