60 On the Parts of Trees 
mal and vegetable life, there is a very obvious analogy be- 
tween some of the organs of plants, and those oi anunals; 
and it does not appear very improbable, that the correspon- 
dent organ, in each, may first fail to execute its office; and 
satisfactory evidence of the imperfect action of any parti- 
cular organ cau much more easily be obtained in the vege- 
table than in the animal world. For a tree may be com- 
posed, by the art of the grafter, of the detached paris of 
many others; and the defective, or efficient, operation, of 
each organ, may thus be observed with the greatest ac- 
curacy. But such observations cannot be made upon 
animals; because the operations necessary cannot be per- 
formed ; and therefore, though there would be much dan- 
ger of error in incautiously transferring the phenomena of 
one class of organized beings to another, I conceive that 
experiments on plants may be, in some cases, useful to the 
investigator of the animal ceconomy. They may direct him 
in his pursuits, and possibly facilitate bis inquiries into the 
immediate causes of the decay of animal strength and 
life; and on a subject of so much importance to mankind, 
no source of information should remain unexplored, and 
no lights, however feeble, be disregarded: 
Naturalists, both of ancient and modern times, have con- 
sidered the structure of plants, as an inversion of that of 
animals, and have compared the roots to the intestines, and 
the leaves to the lungs, of animals; and the analogy be- 
tween the vegetable sap, and animal blood, is very close and 
obvious. The experiments also, of which I have at dif- 
ferent periods communicated accounts to you, supported 
by the facts previously ascertained by other naturalists, 
scarcely leave any reasouable grounds of doubt, that the 
sap of trees circulates, as far as is apparently necessary to, 
or consistent with, their state of existence and growth. 
The roots of trees, particularly those in coppices, which 
are felled at stated periods, continue so long to produce, 
and feed, a succession of branches, that no experiments 
were wanted to satisfy me, that it is not any detective ac- 
tion of the root which occasions the debility-and diseases of 
old varieties of the apple- and pear-tree ; and indeed expe- 
rience every where shows, that a young seedling stock does 
not give the character of youth to the inserted bud or graft, 
I however procured plants from cuttings of some very old 
varieties of the apple, which readily emit roots ; and these 
plants at the end of two years were grafted, about two inches 
above the ground, with a new and very luxuriant variety of 
the same species, These grafts grew yery freely, and the 
roots 
