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the Ascent of the Sap in Trees. 
to put the sap into motion, in this plant, is not definite, but de- 
pends on that to which the plant has been previously accus- 
tomed. Thus, a vine which has grown all the summer in the 
heat of a stove, will not be made to vegetate during the winter 
by the heat of that stove : but, if another plant of the same va- 
riety, which has grown in the open air, be at any time intro- 
duced, after it has dropped its leaves in the autumn, it will 
instantly vegetate. This effect appears to me to arise from the 
latter plant’s possessing a degree of irritability, which has been 
exhausted in the former, by the heat of the stove, but which it 
will acquire again during the winter, or by being drawn out, 
and exposed for a short time to the autumnal frost. On the 
same principle, we may point out the cause why seedling plants 
always thrive better in the spring than in the autumn, though 
the weather be apparently less favourable. In the former sea- 
son, the stimulus of heat and light is gradually becoming greater 
than that to which the plant has been accustomed ; in the latter 
season, it becomes gradually less. 
There is another circumstance attending trees that have been 
made to blossom early in the preceding spring, which has always 
appeared to me an extremely interesting one. If a peach-tree, 
for example, be brought into blossom in one season in the be- 
ginning of February, by artificial heat, it will spontaneously 
shew strong marks of vegetation at the approach of that season 
in the succeeding year ; and, if it be not well protected, it will 
expose its blossoms to almost inevitable destruction. I do not 
see any cause to which this effect can be attributed, except to 
the accumulated irritability of the plant. 
That heat is the remote cause of the ascent of the sap, cannot 
I think be doubted ; and perhaps frequent variations of it are, 
