1885.] Pear Blight and its Cause. 1181 
plant is doubtless largely due to simple displacement as multipli- 
cation takes place, although aided by the limited activity of the 
organism and the movement of the sap. 
The change induced in the tissues of the tree partakes of the 
nature of a true fermentation. That we do not have to do with 
a putrefactive change is patent enough, as no offensive odor is 
given off. The disengagement of carbon dioxide may be made 
evident by partially filling a test tube with fragments of a freshly 
blighted limb and adding water enough to little more than cover 
them. In a short time bubbles of gas will be set free, and a 
drop of lime water held in the tube will show the presence of 
carbon dioxide by the whitening of its surface. It is not so easy 
to determine the nature of the other products formed. Careful 
and repeated tests give no indication of the occurrence of butyric 
acid. The presence of alcohol in very slight amount was shown 
by means of the delicate iodoform test. For this a nearly full- 
grown Bartlett pear, which was thoroughly permeated with the 
blight bacteria from an inoculation made a week before, but the 
tissues still undiscolored, was distilled and the distillate used for 
the test. The amount of alcohol found was so slight, however, 
requiring a microscope to find the crystals produced by the test, 
that it did not account for the main bulk of the product of the 
organism’s activity. This product is presumed to be mainly gum 
of some sort, for the following reasons, tnter alia: the solubility, 
adhesiveness, taste or rather tastelessness and the appearance 
upon drying. There is much probability, indeed, that this be- 
longs to the viscid fermentations, which have been but little inves- 
tigated. 
The bacteria in the tree first attack the starch of the cells, then 
the cellulose of the cell walls, and finally the whole tissue be- 
comes a liquid mass. When, however, the cell walls consist of 
lignin or other secondary substances they are not broken down, 
The action on the cell wall is best seen in the tender, unmodified 
tissues of the fruit. 
After this survey of the characteristics of the organism and the 
chemical changes which it induces, it will be profitable to learn 
something of the nature of the disease itself. The usual impres- 
sion has been that the disease is like a blast of superheated air 
_ passing over an orchard, leaving dead and blackened foliage in 
its track, or that it appears suddenly as the result of a thunder 
