28 



the power of absorption ; for when the vessels arc 

 once filled with fluid, if none be carried off, no 

 more can enter. The absorption is carried on chief- 

 ly by the extremities of the roots, and the trans- 

 piration by the surfaces of the leaves. Dr Hales 

 found, that a sunflower, three and a half feet high, 

 lost by perspiration, on an average, 1 Ib. 14 oz. of 

 fluid ficr day. During a warm night, and without 

 dew, it lost only 3 oz. : when the dew was sensible, 

 it suffered no perceptible loss ; and if the dew was 

 great, or rain fell, it gained a few ounces in weight. 

 The surface of all the leaves of this plant was equal 

 to about 39 square feet, and that of its roots to 15.8 

 square feet. The depth of water absorbed by the 

 whole surface of the roots was ^ 7 part of an inch, 

 and the depth perspired by the surface above ground 

 y~ part. He found likewise, that different plants 

 varied greatly in the quantity of fluid which they 

 perspired ; that evergreens perspired much less than 

 other plants ; and that the perspiration was propor- 

 tional to the extent of surface of the leaves, and 

 ceased nearly if the leaves were removed *. The 

 experiments of M. Guichard shew, that it is chiefly 

 by the upper surfaces of the leaves that this transpi- 

 ration of fluid is carried on ; and those of the same 

 author, of Du Hamcl and Bonnet, demonstrate like- 

 wise that absorption, as far as the leaves are con- 

 cerned, is performed by their under surface. To 

 prove this, the superior surface of one leaf and the 

 inferior surface of another were covered with var- 



* Statical Essays, vol. i. pp. 4. 6. 21. & 28.. 



