Repository: UIHistories Project: Board of Trustees Minutes - 1880 [PAGE 73]

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71 that the virus is harmless on the external surfaces of sound plants; neither does it appear to gain access to the inner tissues through the stomata, or breathing pores. In experiments numbered nine and ten, limbs with the disease, in an active state, were cut and bound in among the branches of healthy trees. No injury ensued. This was upon the first day of July, when the earth was wet and air moist from the heavy rain of the day before. Eight days afterward another storm occurred, the dead branches being left in the trees. Here again, disease might have occurred through inoculations by insects, etc.; but the failure further supports the non-communicability of the disease from without. The circumstances would seem sufficiently favorable for positive results, were these possible. Gf the total number of experiments, of which records were kept, detailed in the accompanying table, thirty-four and seventy-eight hundredths per cent, were effective, while of the whole number of inoculations with knife and needle, fifty-two and seventeen hundredths per cent, unmistakably communicated the disease. Thrown into tabular form the per cents., as now given, are as follows :

N u m b e r of experiments. 36 29 4 69 K i n d s of t r e e s . Virus from pear. 54.00 30.00 100.00 Virus from apple. 72.00 None. None. Successful Successful er m i n o c u l a t i o n s . e x pall i k i e n tss. of nd 63.00 30.00 100.00 52.17 * 38.80 20.69 100.00 34.78

Pear. Apple. Quince. All k i n d s .

The close observations required for the work as now detailed, together with much attention given to the disease during some years, have convinced me that two popular opinions concerning the progress of this fatal disease are not founded upon facts. One is the usual supposition that destruction by this blight is very sudden; that a healthy tree may, in the course of twenty-four hours, become so affected that the leaves will speedily blacken and the branch as speedily die. It is thought that a large part, or all the tree is affected at once, and death follows as it might from a stroke of lightning. I find the march of the destroyer very irregular, but always slow! The difficulty usually is to tell after the lapse of a single day whether any advance has been made, not how much. In experiment numbered seven, the progress, counting both ways from the point of inoculation, was about an inch per day for seventeen days, but this was altogether exceptional. Infection after inoculation rarely shows at all under nine days, and after this time the destroyer seldom gains more than one-fourth of an inch per day in its gradual progress through the cells. But if guided solely by the change of color in the leaves, other conclusions would be reached. Ordinarily the leaves do not become blackened until about two weeks after the infection of the bark at their insertion, except in the case of very young shoots. In number seven, just mentioned, ^very leaf was fresh and green in appearance at the end of the