208 BOTANICAL GAZETTE [APRIL 
varieties are assembled, then the 6, and then the c samples. In 
each case the three (in some cases two) resistant varieties are given 
first, then the non-resistant, in order to facilitate comparison. A 
brief discussion of each factor will be given. 
SPECIFIC GRAVITY.—Referring to the top group of graphs in 
fig. 3, it will be seen that in most cases the specific gravity of the 
expressed juice decreases from a to 0, that is, in the sound fruit 
during storage in the laboratory, and that there is a still further 
decrease from 0 to c, that is, in the rotting fruit. In many cases 
the decrease in the rotted samples is very marked. Probably res- 
piration consumes sugar in the sound fruit with a consequent 
decrease in density of juice, and in the rotting fruit the added 
respiration of the invading fungus causes a still further drop in 
density. STEVENS and Hawkins (43) noted a similar phenomenon 
in rotting strawberries. There is one marked exception to this, in 
the case of the third stage of the Compass variety, but this may be 
an analytical error. 
In fig. 5 there are indications of varietal differences in juice 
density that may be correlated with resistance properties. Thus 
varieties 9, 16, and 30 (resistant) in most cases have a higher 
specific gravity than varieties C and SCF, and this holds even in 
the rotted samples. Whether the osmotic pressure of the host 
sap may be a controlling factor for this fungus is not known. The 
writers know of no measurements of its tolerance to strong nutrient 
solutions except the work of HAwxmns (30), who tested the ability 
of a number of fungi to grow on concentrated solutions of sugars 
and salts. S. cinerea would grow on 2.4 M glucose, 1.4 M potas- 
sium nitrate, and 0.6 M calcium nitrate. These figures would — 
correspond roughly to 43, 14, and 9 per cent, respectively, which 
are far higher than any concentrations of fruit juices. It is to be 
noted, however, that HAWKINS gives no information as to the rate 
of growth at the various concentrations used; hence it is possible 
that the differences found in the saps of the plum varieties might 
account in part for the differences in rate of growth of the fungus. 
HYDROGEN-ION CONCENTRATION.—In fig. 3 a comparison of the 
changes in reaction of the juice can be made, the values being 
given in:terms of P,. No consistent and striking differences are 
