FUNCTION.] MOTION OF FLUIDS. 323 



CHAPTER XVI. 



OF THE MOVEMENTS OF FLUIDS. 



PLANTS have no circulation of their fluids analogous to 

 that of 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 water 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, as has 

 been already seen, 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 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} 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. 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 than in deciduous or 



Y 2 



