Nov. 39. 1915 Varietal Resistance of Plums to Brown-Rot 389 



present (PI. XXXVII, fig. 6). The plums at the time of preserving the 

 material (Nov. 7, 19 14) were firm. An examination of the healthy 

 tissue of ripe susceptible varieties revealed the fact that the middle 

 lamella in these was completely dissolved (PI. XXXVII, fig. 5). These 

 plums were soft when the material was fixed. That the pectic-acid 

 compounds change to pectin in the ripening fruit is a well-known fact. 

 In view of the fact that the brown-rot can only spread after the middle 

 lamella has been dissolved, the reason for the increase in susceptibility 

 on ripening in those varieties which become soft as a result of the normal 

 loss of the middle lamella owing to ripening is readily seen. 



The reduced possibilities of infection owing to the plugging of many 

 of the stomata, the causes of which have already been explained, and 

 the persistence of the middle lamella after ripening, as shown by the 

 fact that the fruits remain firm, explain the resistance to brown-rot of 

 such varieties as Reagan, BXW15, BxWg, S. D. Nos. 2 and 3, and 

 Americana Seedling No. i. 



RELATION OF TANNIN CONTENT OF THE HOST TO RESISTANCE 



A great deal of attention is being given to the relation between chemical 

 substances within the host cell and resistance. The work of Comes (i 91 3) 

 on the correlation between the increased acid content in wheat plants 

 and rust resistance has been mentioned. Cook and Taubenhaus (1911) 

 were able to show that tannin, a very common product in plants, was 

 toxic in varying degrees to many fungi in culture and considered that it 

 might be a very important factor in resistance. Bassett and Thompson 

 (1911) showed that apples and pears contain an oxidizing enzym capable 

 of producing from gallic acid a tannin-like substance having the power 

 of precipitating protein from solution. They found this product to be 

 toxic to "a fungus." The juices of green apples, pears, and walnut hulls 

 (unboiled) produced a substance which on standing precipitated soluble 

 protein from the juice. They considered this to be a tannin-like sub- 

 stance and to be controlled by the oxidizing enzym. 



If the tannins disappear on the ripening of the fruit, as is generally 

 supposed, we may have an explanation of the greater susceptibility of 

 some fruits to disease on ripening. The evidence of the disappearance 

 of tannin on ripening, however, is not at all conclusive. One of the 

 most striking instances' of its apparent disappearance is that of the per- 

 simmon {Diospyros virginiana), the green fruits of which are very astrin- 

 gent, while the ripe, soft fruits are not at all astringent. Gore (1911), 

 however, showed that the tannin did not disappear, but was inclosed in 

 sacs which broke readily in green fruits in contact with saliva, but were 

 not affected in the ripe fruit. Similar structures have been observed in 

 the carob-bean pod {Ceratonia siliqua) and in the date fruit. Bassett 

 and Thompson (191 1) demonstrated that "apples that had fallen from 

 9842°— 15 3 



