No. 83] 365 
the bacteria will not convey the disease. Until this demonstration 
is furnished, and it is one that 1s entirely feasible,the bacterial 
theory of pear blight is not thoroughly proven. But nevertheless 
it is quite evident that, as the theory is in accord with all the known 
facts, part of which are otherwise unexplainable, it answers for the 
present every practical requirement. 
Taking it for granted now that the disease is due to bacteria, I 
shall pass to the discussion of its natural mode of propagation from 
tree to tree. The experiments performed by Professor Burrill con- 
sisted in bringing diseased branches in contact with healthy ones, 
aud in smearing the surface of twigs and leaves with virus; they 
gave no results. I repeated the latter upon green fruit, without re- 
sults. It was noticeable in the case of the ulcerous pears that the 
pus, as it ran over the surface of the yet unaffected part, did not dis- 
color or change the tissues in the least. In trying the experiment 
upon leaves I was able, in one instance, to communicate the disease 
to a vigorous growing shoot by smearing virus upon a very young 
leaf at the apex. This was done in the laboratory by keeping the 
eut end of the shoot in water. In another experiment a watery in- 
fusion of the blight, such as used for inoculation, was permitted to 
drip for some hours at about the rate of four drops per minute upon 
a tender shoot kept fresh in water, care being taken that none of the 
infusion got into the water in which the shoot was placed. Both 
apple and pear shoots were tried, and one of the former appeared to 
take the disease. But none of these experiments were satisfactorily 
performed, and I only mention them as affording serviceable hints. 
In another trial a young pear tree about a foot and a half high, which 
had become well established in a flower pot, was watered exclusively 
from August 7 to 31 with a copious supply of water made milky 
with infusion of pear blight; no results followed. 3 | 
From such scanty data as these no positive conclusions can be drawn, — 
but the following conjectures seem plausible: The bacteria escape 
from the tissues in the slimy drops that ooze out from the diseased 
parts, especially in damp weather. They are washed off and freed 
from the viscid part by rains, and upon becoming dry are taken up 
by the winds. Being now suspended in the air, a damp day, dewy 
night, or light rain would bring them in contact with the delicate 
surface tissues of expanding buds, or the exposed internal tissues of 
fresh cracks or wounds, in the most favorable way to introduce the 
contagion. This is quite in accordance with the fact that the disease 
usually starts at the ends of the branches, but also appears sometimes 
on the larger limbs and even the trunks. It also explains the fact 
that the rankest growers are most subject to attack, these exposing 
more tender surfaces, and upon the disease obtaining a foothold, fur- 
nishing more succulent tissues. The disease is even said by some to 
be a product of high cultivation ; on the other hand, a nurseryman 
told me a few days since that the land we were surveying was so 
poor that pears would not blight upon it, which I could believe was 
quite possible, for the trees would grow so slowly that no surface of suf- 
