389 
CHAPTER XIII. 
OF THE CIRCULATION. 
Plants have no circulation of their fluids analogous to that 
of the blood in the higher animals ; that is to say, departing 
and returning incessantly from and to one common point. 
But that their fluids have a motion may be inferred from 
their nature ; and that it is often of extreme rapidity is proved 
by the great quantity of vrater which they perspire; all of 
which must be replenished by aqueous particles in rapid 
motion along the tissue from the roots. A young vine leaf, 
in a hot day, perspires so copiously, that, if a glass be placed 
next its under surface, it is presently covered with dew, which, 
in half an hour, runs down in streams. Hales computed the 
perspiration of plants to be seventeen times more than that 
of the human body. He found a sunflower lose one pound 
four ounces, and a cabbage one pound three ounces, a day by 
perspiration. By some contrivances of glass tubes and a mer- 
curial apparatus, he found means to measure the force of 
suction in particular trees, which will of course be in propor- 
tion to the amount of evaporation ; and he ascertained that 
an apple branch 3 feet long would raise a column of mercury 
5| inches in half an hour ; a nonpareil branch 2 feet long, 
with 20 apples on it, 12 inches in 7 minutes; and the root 
of a growing pear tree 8 inches in 6 minutes. In short, he 
computed that the force of motion of the sap is sometimes 
five times greater than that which impels the blood in the 
crural artery of the horse. Guettard asserts that the young 
shoots of Cornus mascula lose twice their own weight a day. 
This perspiration is regulated in part by the number of the 
stomates, and in part by the thickness of the epidermis : hence 
evergreens, in which the stomates are small, and less numerous 
c c 3 
