Nov. 29,1915 
Varietal Resistance of Plums to Brown-Rot 
389 
present (PI. XXXVII, fig. 6). The plums at the time of preserving the 
material (Nov. 7, 1914) 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, BXW9, S. D. Nos. 2 and 3, and 
Americana Seedling No. 1. 
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 (1913) 
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 sttiqua) and in the date fruit. Bassett 
and Thompson (1911) demonstrated that “apples that had fallen from 
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