280 
tree, while none other will, may never be fully known, but 
some of the controlling conditions are undoubtedly within 
the range of discovery. : 
Several investigators have expressed the opinion that the 
acidity of plant tissues prevents, to a large extent, the 
growth of parasitic bacteria, for most forms are very sensi- 
tive to acids. This has suggested the acidulation of artificial 
cultures. From a limited number of cultures it was learned 
that one-half of one per cent of malic acid added to potato 
solution prevented turbidity and the formation of a pellicle. 
The chief microscopic evidence of growth in this case was 
alight fleecy sediment, which was found to be composed 
- entirely of blight bacteria. Increasing the amount of malic 
acid to two per cent., only diminished the growth of the 
bacteria, without entirely arresting it. With five per cent. 
of citric acid some growth took place, but none at all with 
the same amount of tartaric acid. 
The experiments show that the blight bacteria can grow 
in the presence of a relatively large amount of fruit acids. 
This corresponds with preconceived opinions, and with the 
fact that the normal juices of the pear and apple are strongly 
acid to test paper. It is also found that the expressed and 
filtered juices of green pears make excellent culture liquids, 
without neutralizing. 
From the beginning of these studies both observation and 
experiment have pointed to a connection between the activity 
of the disease and the succulency of the tissues.1_ The in- 
ference has been chiefly founded upon the certainty and 
rapidity of the disease when inoculated into more succulent 
tissues, aud from the behavior of the bacteria in fresh 
pear fruit supplied with a varying amount of water. It is 
well known to every horticulturist that a rapid, succulent 
growth, brought about by high cultivation, or by any other 
means, is the condition which favors the disease. In 1866 
Mr. W. C. Flagg? called attention to the percentage of water 
in the heartwood, sapwood and bark of the apple and pear, 
determined by Emmons? as follows : 
Heartwood. Sapwood. Bark. 
PEAT soho wieetaaeice detalles marae nt td sey 22.05 48.80 63.70 
AD plas tits tt ALO Fat) Peri Caan Orie 83:85 | 39/10/59 
He inferred from these figures that ‘‘in the less density 
of pear wood and the greater amount of water in its sap- 
wood, we have probably one of the conditions of its greater 
liability to blight.” Summarizing the data of Emmons we 
learn that the pear contains one per cent. more water than 
the apple. 
1Report for 1884, p. 362; for 1885, p. 246. 
2Proc. Mo. Hort. Soc. for 1866, p. 401. 
*Rep. U. 8. Pat. Office for 1849, p. 475. 
