248 
alcoholic fermentation. Another test was carried out for me by 
Dr. Babcock, the chemist of the Station. A freshly blighted pear: 
was distilled and the distillate tested by the delicate iodoform pro- 
cess, which consists in adding a little potassic hydrate, boiling and. 
again adding a potassic iodide solution of iodine.* Had there been. 
common alcohol, butyric alcohol or acetic ether present in quantity 
commensurate with the amount of material used and extent of the 
change in it, a yellow precipitate would have been seen; but it was ab-- 
sent. To make doubly sure, after allowing sufficient time for subsid- 
ence, a drop was taken from the bottom of the liquid and put under 
the microscope; a very few characteristic hexagonal crystals were thus. 
found. A trifle of alcoholic fermentation had evidently taken place. 
during the blighting, but not so much but that it might well be ac- 
counted for by the accidental presence of some true alcoholic ferment 
in minute quantity. A control test was made by distilling a freshly 
gathered, sound pear, and treating it in the same manner as in the 
other case; no trace of crystals could be found. A test of blighted. 
pear with Fehiing’s solution gave reaction for glucose, but not more: 
than one would get with araw potato. After all, while there may be: 
some alcoholic or even putrefactive changes taking place during ac- 
tive blight, the total product of the change is far too great to be ac-. 
counted for by either or both of these together. The substance. 
which is obtained by the action of the blight bacteria on the starch, 
unlignified cell walls and other substances of the plant tissue, is a. 
very viscid, creamy matter, soluble in water, and drying to a hard 
gum on exposure to theair. Without further explanation it may be 
said that so far as observation goes, and so far as tests have been 
applied, this seems to be one of the viscous fermentations, a class: 
whose physiology and chemistry has been but little studied, and the. 
investigation of which is beset with unusual difficulties. 
To sum up the conclusions to be drawn from the work of the year: 
pear blight, according to rigid proof, is due to a specific kind of bac- 
teria; the germs for the most part gain entrance to the tissues of the 
tree early in the season through the tender surface tissues of flowers. 
and new shoots; the disease passes through a longer stage of incuba- 
tion than heretofore recognized ; some peculiarities of the disease: 
may be explained by the varying per cent. of sap in the tissues, but. 
the resistance and susceptibility of different varieties has not yet 
been formulated; the disinfection of the pruning knife is practically 
unimportant ; the germs may grow in many kinds of dead organic 
matter outside the tree, and on again gaining entrance to it be able 
to set up the disease in its full virulency; no formation of spores or 
other transformations except the production of zoogloea have yet 
been detected; the chemical changes induced by the bacteria in the 
plant tissues are in part the formation of a gum and disengagement 
of carbon dioxide, but not of alcohol, butyric acid or any of this se- 
ries of compounds, at least to any important extent; and lastly the 
change is not putrefactive or alcoholic, but in all probability vis- 
cous. 
e 
*For exact method cf. Prescott’s Proximate organic analysis, p. 177. 



