390 
Journal of Agricultural Research 
Vol. V, No. 9 
the tree showed about twice as much tannin as those freshly plucked.” 
It is a matter of common observation that some plums, especially the 
sand cherry, contain considerable amounts of an astringent substance, 
probably tannin, even when dead ripe. It is not altogether clear, there¬ 
fore, that the disappearance of the tannin on ripening is a cause of the 
increased susceptibility of ripe fruits to rot. 
There is still the possibility that differences in resistance of varieties 
may be due to unequal tannin content. In order to determine this point, 
tannin determinations were made of the fruit of n varieties of plums. 
The method used was Proctor’s modification of Lowenthal’s method as 
described by Leach (1913, p. 370). The results given in Table V are for 
tannin substances calculated as gallotannic acid. The determinations 
were made on fruit which had been picked 14 hours, except in the case 
of the sand cherry and Compass, which were made directly after picking. 
Table V .—Tannin content of ripe and green plums on August 6, IQ15 
Variety. 
Condition. 
Date of 
ripening. 
Percent¬ 
age of 
tannin in 
pulp. 
Percent¬ 
age of 
tannin in 
dry 
matter. 
Percent¬ 
age of 
dry 
matter. 
Relative sus¬ 
ceptibility. 
Sand cherry . 
Ripe . 
Aug. 
1 
2. 087 
IS- 081 
13.84 
H—1—1—f- 
131 X (sand-cherry 
•234 
1.4S3 
iS- 75 
+ + + + 
hybrid). 
CompassXpin cherry. 
• 338 
2.388 
14.17 
+ + + + 
Sapa . 
Turning. ... 
Aug. 
17 
.362 
3 - 367 
i °-75 
+ + + + 
Compass . 
Green . 
Aug. 
15 
• 483 
4 . 229 
II. 42 
+ + + + 
A X W12 . 
...do . 
. 482 
2. 418 
I4. IO 
+ + + 
Opata . 
Turning. ... 
Aug. 
17 
• 733 
O’ T" 
4. 6l8 
IS- 87 
+ 4 - 4 - 
Burbank . 
Green . 
... do. 
. 185 
I. 516 
12. 20 
+++ 
B X W21 . 
Aug. 
19 
•773 
5-777 
13 * 38 
++ 
A X W15 . 
...do. 
Sept. 
2 
1.131 
9. 520 
11.88 
++ 
Americana Seedling 
.665 
3*873 
17. 17 
+ 
No. 1. 
The relative-susceptibility determinations were made at the same time 
as the tannin determinations and are confirmed by previous tests on some 
of the varieties and by field observations on all of them. 
It is readily seen that very little relationship exists between tannin con¬ 
tent and resistance to the brown-rot fungus. Even though a correlation 
could be shown between tannin content and resistance, it still remains 
to be proved that the tannin is an actual factor in resistance, since the fol¬ 
lowing facts indicate that it does not come into direct contact with the 
fungus hyphse. The hyphae are apparently always intercellular, and 
according to Haas and Hill (1913, p. 192)— 
In the cell the tannin occurs in solution in the cell sap, and since tannin forms a 
precipitate with albuminous matter it follows that the layer of protoplasm around the 
tannin vesicles must be impermeable to it; if this were not so the protoplasm would 
be tanned on the production of tannin. 
