CHAPTER XL 
OF THE CIRCULATION OF THE SAP. 
Plants have certainly no general circulation of their juices 
analogous to that of the blood in animals, — that is to say, 
departing and returning incessantly from and to one common 
point. But that their fluids have a motion often of extreme 
rapidity, is proved by the great quantity of water which they 
perspire ; all of which must be replenished by aqueous parti- 
cles 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 sun-flower 
lose one pound four ounces, and a cabbage one pound three 
ounces, a day by perspiration. By some contrivances of glass . 
tubes and a mercurial apparatus, he found means to measure the 
force of suction in particular trees, which will of course be in 
proportion to the amount of evaporation; and he ascertained 
that an apple branch 3 feet long would raise a column of 
mercury 5 J 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 cuticle : 
hence evergreens, in which the stomates are small, and less 
numerous than in deciduous or herbaceous plants, and the 
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