62 On the Parts of Trees primarily impaired ly Age. 
I have endeavoured, in several former communications, 
to prove that the sap of plants circulates through their 
leaves, as the blood of animals circulates through their 
Jungs; and I have not subsequently found any facts, in the 
writings of other naturalists, or in my Own experiments, 
which militate against this conclusion. I have also ob- 
served, that grafted trees, of old and debilitated varieties 
of fruit, became most diseased in rich soils, and when 
grafted on stocks of the most vigorous growth; which has 
induced me to suspect, that in such cases more fouda is col- 
lected, and carried up into the plant, than its leaves can 
prepare and assimilate, and that the matter thus collected, 
which would have promoted the health and growth in a 
vigorous variety, accumulates, and generates disease in the 
extremities of the branches and annual shoots, whilst the 
lower part of the trank and roots remain, generally, free 
from any apparent disease. I am, therefore, much dis- 
posed to attribute the diseases and debility of old age in 
trees, to an inability to produce leaves, which can efficiently 
execute their natural office; and to some consequent im- 
perfection in the circulating fluid. It is true that the leaves 
are annually reproduced, and therefore annually new: but 
there is, I conceive, a very essential difference between the 
new Jeaves of an old, and of a young variety: and in sup- 
port of this opinion, I shall observe, that the external cha- 
racter of the leaf of the same variety at two, and at twenty 
years old, is very dissimilar; and it therefore appears not 
improbable, that further’ changes will have taken place at 
the end of two centuries *. 
If these opinions be well founded, and the Jeaves of trees 
be analogous to the lungs of animals, i is it very improbable 
that the natural debility of old age of trees and of animals, 
may originate from a similar source 2—This is a question, 
upon which I am not by any means prepared to give an 
opinion: but I believe it will very generally be admitted, 
that the human subject is best formed for long life, when 
the chest is best formed to permit the lungs to move with 
most freedom. I have also long and attentively observed 
* ‘The leaf of a seedling apple or pear-tree, when the plant is very young, 
is generally almost wholly free from the pubescence or down, which subse- 
quently appears on its under surface; and which Bonnet and M. Mirbel 
have supposed to increase its surface aud powers. But I feel little disposed 
to adopt this hypothesis, having observed that the leaves of some new va- 
rieties of the apple, which have sprung from sceds of the Siberian crab, have 
both surfaces nearly equally smooth; and that these varieties grow Faster, 
and bear heavier crops of very rich fruit, than any others, without being 
exhausted or injured, 
amongst 
‘ 
