hi 
1885.] Pear Blight and its Cause. 1185 
germs cause the disease, but that they are merely accompaniments 
of it. To meetthis objection, and place the subject upon a logical 
and irrefragible basis it is necessary to state the results of still 
further studies. In order to determine whether other bacteria 
will grow under the same circumstances, various kinds were in- 
oculated into pear trees—bacteria from rotting spots in green 
tomatoes, from various sorts of putrefactions, those which had 
incidentally appeared in various culture experiments—and uni- 
formly with negative results. When inoculation was made from 
a culture of blight bacteria contaminated with other forms, the 
resulting blight contained but the one sort. It is a well-known 
fact that most bacteria will not thrive in acid solutions, and 
Hartig has supposed that the reason that plants are so free from 
parasitic bacteria (only two, or at the most three, true vegetable 
parasites being known among them) is that they cannot withstand 
the acidity of the sap. Be this as it may, only one form of bac- 
teria has yet been found to accompany pear blight. 
But this does not dispose of the possibility that the blight is 
not caused by the bacteria, but by some deleterious substance 
which goes with them or which they produce. It is obvious that 
as the blight may be produced by using a drop of water which 
has been flowed over blighted tissues, the active agent must be 
either the bacteria or the substances which the water dissolves. 
. There is a very simple way of separating solutions from bacteria 
by filtering through porous earthenware, which permits the fluid 
to pass, but not the bacteria. It has been demonstrated by trial 
that inoculating from a filtrate prepared in this way will not pro- 
duce the blight. Separating the bacteria from all accompanying 
substances is accomplished by means of fractional cultures. Such 
a drop as used for inoculating is introduced into a suitable steri- 
lized culture fluid; after some days, when the bacteria have well 
filled it, a drop is removed and used to start another culture, and 
so on. In this way the bacteria are kept vigorous by growth and 
multiplication, and the unvitalized substances which accompanied 
them in the first drop are more and more diluted at each transfer. 
Finally a drop from the last culture of the series, in which the 
amount of substance derived from the original drop must be so 
infinitesimally small as to be inoperative, is used to inoculate with 
again. Carefully conducted experiments of this kind have given 
as severe blight as in direct inoculation. No stronger proof is 
needed that the bacteria are solely responsible for the disease, 
