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 ii 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. 



Sand cherry 



131 X (sand-cherry 



hybrid). 

 Compass X pin cherry, 



Sapa 



Compass 



AX W12 



Opata 



Bnrbank 



BXW21 



AXW15 



Americana Seedling 



No. I. 



Ripe. 

 ..do. 



..do.... 

 Turning. 

 Green . . 

 ..do.... 

 Turning. 

 Green . . . 

 ..do.... 

 ..do.... 

 ..do.... 



Date of 

 ripening. 



Aug. 



Aug. 17 

 Aug. 15 



Aug. 17 



..do 



Aug. 19 

 Sept. 2 



Percent- 

 age of 



tannin in 

 pulp. 



Percent- 

 age of 

 tannin in 



dry 

 matter. 



15. 081 

 1.483 



2.388 

 3-367 

 4. 229 

 3.418 

 4.618 

 1.516 

 5-777 

 9.520 



3-873 



Percent- 

 age of 

 dry 

 matter. 



13.84 



15-75 



14. 17 



10-75 



11. 42 

 14. 10 

 15-87 



12. 20 

 13-38 

 11.88 

 17. 17 



Relative sus- 

 ceptibility. 



+ + + + 



+ + + + 



+ + + + 



+ + + + 



+ + + + 



+ + + 



+ + + 



+ + + 



+ + 



+ + 

 + 



The relative-susceptibiUty 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 arotuid the 

 tannin vesicles must be impermeable to it; if this were not so the protoplasm would 

 be tanned on the production of tannin. 



