— 6 — 
climate, yet was the pearblight never heard of until recently.’' 
The prevalence of blight in 1845 ascribed to a frost 
occurring on the tenth of May. This writer cites a much 
more severe frost on the same day of the year 1834, but 
there was no blight that year. 
A further objection refers to the effect of frost upon sap. 
“The freezing of sap does not change its properties. That 
the freezing of vegetable matter in a certain state of devel- 
opment produces death, may be admitted.” “It may 
also be admitted that the freezing in winter may be so severe 
as to destroy the vital principle as well in vegetable as ani- 
mal life.” “Death thus produced is not occasioned 
by deleterious properties imparted to the sap, butby the mech- 
anical force of the frost upon the cellular and woody tissues.” 
“All our trees are frozen, except their trunks and 
large branches, every winter, especially the young and ten- 
der wood of the past summer’s growth, and if an elaboration 
of the sap injurious in its consequences were thereby pro- 
duced, no vegetable matter would survive a single winter. 
The economy of the vegetable world rests not on so insecure 
a basis as this would indicate.” This writer here speaks of 
the spread of the disease in the individual plant, and cites a 
case of the production of the disease in a healthy tree by 
inoculation from a diseased tree. Further he says: “There 
is no occasion to theorize upon this subject for the mere 
sake of theory, and I have none that I regard as certainly 
true; but I strongly incline to the belief that the pear blight 
is an epidemic, that it prevails like other epidemics, and will 
pass off like them. The atmosphere is, I believe, generally 
admitted to be the medium by which they prevail, and are 
carried from place to place. What that subtle principle may 
be, which pervades our atmosphere, by which infection is 
retained and transmitted, so that, like the Asiatic cholera, 
it makes the whole circuit of our earth, human science has 
not discovered, and perhaps never will; but that such a prin- 
ciple exists, is sufficiently obivious from its effects.” 
Looking back in the light of what “human science” in 
the modern times has discovered, to those days when the 
germ theory was little more than a suggestion, the statement 
above quoted is of interest. 
CAUSES SOUGHT IN ATMOSPHERIC AND SOIL CONDITIONS. 
The writers for the agricultural press of fifty years ago 
were much inclined to look for causes of the disease in the 
attendant atmospheric and soil conditions. One writer in 
1851 says: * “A fruit tree planted on a well-drained poor 
* Patent Office Report. Agriculture. 1851 pgge 403. 
