14 | ANNUAL REPORT. 2 wpm 
add to what little is now known. AIILI have to offer is theory, and 
this chiefly the theory of others ; but as much as I dislike iy inghsa 
ry, I think it will be as good, at least, as blindly referring the cause 
of the disease in question to some electric condition of the atmos- 
here, : it haan 
P'The blight of the pear tree, which I assume is nearly the same as 
the blight of the apple tree of this section, Downing divides into two 
kinds—lInsect Blight and Frozen Sap Blight. If this division be 
correct, I assume again that the blight of this section is mainly 
Frozen Sap Blight. To explain this, he presumes ‘‘ a damp, warm 
autumn by which the tree is forced into a late growth, succeeded by 
a very sudden and early winter. While the sap vessels are still 
filled with their fluids, a sudden and sharp freezing takes place, or 
is, perhaps, repeated several times, followed, in the day time, by 
bright sun. The descending current of sap becomes thick and clam- 
my, so as to descend with difficulty ; it chokes up the sap vessels, 
freezes and thaws again, loses its vitality, and becomes dark and 
discolored, and in some cases so poisonous as to destroy the leaves 
of other plants when applied to them. Here along the inner bark it 
lodges and remains in a thick, sticky state all winter. If it happens 
to flow down till it meets with any obstruction, and remains in any 
considerable quantity, it freezes again beneath the bark, ruptures 
and destroys the sap vessels, and the bark and some of the wood 
beneath it shrivels and dies. 
‘In the ensuing spring the upward current of sap rises through 
its ordinary channel—the outer wood or alburnum—the leaves ex- 
pand, and for some time, nearly all the upward current being taken 
up to form leaves and new shoots, the tree appears flourishing. To- 
ward the beginning of summer, however, the leaves commence send- 
ing the downward current of sap to increase the woody matter of 
the stem. ‘This current, it will be remembered, has to pass down- 
ward through the inner bark or liber, along which still remain por- 
tions of the poisoned sap, arrested in its course the previous au- 
tumn. This poison is diluted, and taken up by the now downward 
current, distributed toward the pith, and along the new layers of al- 
burnum, thus tainting all the neighboring parts. Should any of the 
adjacent sap vessels have been ruptured by frost, so that the poison 
thus becomes mixed with the still ascending current of sap, the 
branch above it immediately turns black and dies, precisely as if 
poison were introduced under the bark. And very frequently it is 
accompanied with precisely the odor of decaying, frost-bitten vegeta- 
tion.” 
Thomas does not adopt any theory. He says, ‘‘ but after admit- 
ting that the different theories may be in part correct, and that the 
blight may be caused by a combination, in a greater or less degree, 
of each assigned cause, we are driven to the conclusion, from a 
large number of observations of which these limits preclude even a 
brief recital, that the cause of blight, like that of potato disease, re- 
mains hidden in a large number of instances, from our knowledge. 
And that whether the latent tendency to disease is only increased 
and developed by changes of the weather, or whether those changes 
actually produce them, is yet enveloped in doubt.” 
