287 
The fact that this difference far exceeds the greatest dif- 
ference yet found between like parts of the most diverse 
varieties is, indeed, suggestive. To state it in another 
way—the greatest variation yet found between varieties of 
the pear in regard to succulency is much exceeded by the 
variation between the parts of the same tree. | 
Chemical changes.—Only an imperfect understanding yet 
exists of the chemical transformation brought about in the 
tree by the activity of the blight bacteria. The year’s work 
has contributed somewhat to the subject. 
To determine if the sugar of the plant was increased 
or decreased by the bacteria, the following tests were made, 
with the assistance of my colleague, Mr. Ladd. Immature 
pears, thoroughly permeated with blight from inoculations 
made eight days before (June 28), were contrasted with 
sound pears taken from the same tree at the same time and 
subjected to like treatment. Four or five pears of each 
kind were taken, the sound ones weighing 141.1 grams, 
and the blighted 111.6 grams. The sound fruit yielded 3.94 
per cent. of fruit-sugar, and the other 1.61 per cent., show- 
ing a large loss of sugar to be charged without doubt to the 
action of the bacteria. 
The solutions used in the preceding analyses, which were 
formed by digesting the material in alcohol, afterwards 
rinsing in water, and to the combined solutions adding 
enough water to reduce the alcohol in the solution to less 
than 20 per cent., were tested viscometrically by my col- 
league, Dr. Babcock. The result! gave 156 for the sound 
and 280 for the blighted fruit. These numbers represent 
the number of grams of sugar which it would be necessary 
to add to a litre of water to secure the same viscosity. By 
referring to the percentages of sugar already found in these 
same solutions, it will be seen that the solution from the 
sound fruit contained 39 grams of sugar to the litre, and 
the other 16 grams. Subtracting these numbers from the 
viscosities, leaves 117 and 264 as the respective indices of 
sound and blighted fruit when the sugar is eliminated. The 
substance which may be regarded as chiefly increasing the 
viscosity of a solution from blighted fruit over and above 
that from sound fruit is probably the gum or gum-like sub- 
stance formed by the bacteria. The difference between 117 
and 264, 7. e., 147, may be taken to represent the viscosity 
of this substance in blighted fruit. This number is so large 
that evidently only a small part of the gum is derived from 
the conversion of sugar, probably not even a fifth. It has 
been conjectured that starch contributes most largely to 
the formation of the gum, but although this is probable, 
there is no direct proof of it. 


1For the method of procedure see the chemist’s report in this volume. 
