PLANTS. 
17 
should seem, to have made it get forwarder than a 
branch belonging to a tree, whose roots, trunk, and 
all its other branches, were at the very time frost- 
nipped. Notwithstanding this, the branch did not 
seem affected by the state of its trunk ; but the ac- 
tion of the heat upon it produced the same effect 
as if the whole tree had been in the hot-house.” 
In France we find Duhamel and Delahire con- 
tending for the regular and uniform circulation of 
this fluid in vegetables ; and Mr. Knight, in some of 
the late volumes of the Philosophical Transactions, 
has supported an opinion to the same effect. 
As soon as the spring commences, if the frost has 
left the ground, the sap begins to rise in plants, and 
that with such force, when the weather is favourable, 
that the cut end of a vine branch has supported a 
column of mercury 32 a. inches high. There can be 
no doubt that the sap flows from the root of the tree 
to the very extremity of its branches; for, if a num- 
ber of openings be made in a tree when the sap is 
beginning to rise, it will first appear at the lowest 
wound, then at the next to the lowest, and so on 
successively, till it flows from the highest of all. 
This was proved in a pleasing manner by some fo- 
reign philosophers, who made plants vegetate in 
coloured liquors, and found that the wood became 
coloured as the sap rose, till at length it reached 
the top of the tree and tinged the very leaves. 
A great part of the sap, after it has flowed to the 
leaves, is carried off by perspiration, and the quan- 
tity perspired has been proved, by several philoso- 
vol. hi. e 
