188 BOTANICAL GAZETTE [warce 
tion of a living organism of fungous or bacterial nature with the 
diseased condition, no matter how intimate or constant such 
association may be. Of course, there may be cases, such asthe — 
| 
‘‘black-knot” of plum trees, in which the effect of the fungs 
is so apparent and its parasitic nature so manifest that the ev 
dence of the unaided eye is almost conclusive (though even it 
this case it will be remembered that for years the knots wee 
supposed to be caused by insects), but in the vast majorityad 
cases a far more searching proof is necessary. The organisa — 
whose parasitic nature is in question must be isolated from it 
host and grown in a pure culture; thence it must be transfer’ 
with due care to the uninjured tissues of a healthy plant of the 
same species as that from which it was derived, growing unde 
normal conditions; in this plant it must produce symptoms of | 
disease identical with those originally observed; and, finallj, 
from this plant the same organism must again be isolated. Only 
under the fulfillment of such conditions can an organism be 
stamped as an absolute parasite. 
These are rules made familiar to us by the methods of moder | 
bacteriology, but they too seldom enter into the practice of the ' 
vegetable pathologist. It may be said in passing that their 
ment cannot always be attained. It is more than probable 
only an extremely small proportion of the diseases of plants w 
are commonly attributed.to fungous parasites are absolutely ea 7 
sitic in their nature —that is, due to organisms which or q 
and penetrate the uninjured tissues of healthy plants pee | 
under normal conditions, and live therein at the expense a" = 
the detriment of the host. In most cases the pathologist = 
be prepared to search for injured tissues offering an opp? oe 
for saprophytic, followed possibly by parasitic attach 7 
unfavorable surroundings weakening the plant or rendeny : 
peculiarly susceptible to the attacks of semiparasitic ee : 
Such conditions are easily induced, and great care > : a 
exercised in drawing conclusions from results obtain 
laboratory or greenhouse from inoculations of wounded which 
of plants kept under conditions of warmth and noise 
it 
. 
