Mr. Knight on the Parts of Trees , &c\ ijg 
organ can much more easily be obtained in the vegetable, 
than in the animal world. For a tree may be composed, by 
the art of the grafter, of the detached parts of many others ; 
and the defective, or efficient, operation, of each organ, may 
thus be observed with the greatest accuracy. But such ob- 
servations cannot be made upon animals ; because the opera- 
tions necessary cannot be performed ; and therefore, though 
there would be much danger of error in incautiously trans- 
ferring the phenomena of one class of organised beings to 
another, I conceive that experiments on plants may be, in 
some cases, useful to the investigator of the animal economy. 
They may direct him in his pursuits, and possibly facilitate 
his enquiries 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 between 
the vegetable sap, and animal blood, is very close and 
obvious. The experiments also, of which I have at different 
periods communicated accounts to you, supported by the 
facts previously ascertained by other naturalists, scarcely 
leave any reasonable 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 
