THE FREE WATEU OF THE SOn,. 213 



too little, in fact, to l)o estimated by tlic methods hitherto 

 employed. Sachs explains this as follows : Assuming that 

 the roots have at a given temperature as strong an attrac- 

 tion for Avater in the state of vapor as for liquid water, the 

 amount of each taken up in a given time under the same 

 circumstances would be in pioportion to the weight of 

 each contained in a given space. A cubic inch of water 

 yields at 21'2° nearly a cubic foot (accurately, 1,696 times 

 its volume, the barometer standing at 29.92 inches) of 

 vapor. We may then a><sumc that the absorption of liq- 

 uid or hygroscopic water proceeds at least one thousand 

 times more rapidly than that of vapor, a diffei-ence in 

 rate that enables us to comprehend why a plant may gain 

 water by its roots from the soil, when it would lose water 

 by its roots were they simply stationed in air saturated 

 with vapor. 



Again, the soil need not be more hygroscopic than roots, 

 to supply the latter with water. It is important only that 

 it present a sufficient surface. As is well known, a plant 

 requires a great volume of earth to nourish it properly, 

 and the root-surface is trifling, compared to the surface 

 of the particles which compose the soil. 



Boussingault found by actual measurement that, accord- 

 ing to the rules of garden cultuie as practiced near Stras- 

 burg, a dwarf bean had at its disposition 57 pounds of 

 soil; a jiotato plant, 190 pounds; a tobacco plant, 470 

 pounds ; and a hop plant, 2,900 pounds. These weights 

 correspond to about 1, 3, 7, and 50 cubic feet respectively. 



The Quantity of Water in Vegetation is influenced by 

 that of the Soil. — De Sanssure observed that plants grow- 

 ing in a diy lime soil contained less water than those from 

 a loam. It is well known that the grass of a wet summer 

 is taller and more succulent, and the green crop is heavier 

 than that fi-om the same field in a dry summer. It does 

 not, however, make much more hay, its greater weight 

 consisting to a large degree of water, which is lost in dry 



