210 HOW CROPS FEED. 



saturation, furnish the tobacco }>lant with 44.1"!^ of its 

 weiglit of water. 



A coarse sand that could hohl 20.8° ]„ of water was 

 found to yield all but 1.5° ]„ to a tobacco plant. 



From these trials we gather with at least approximate 

 accuracy the power of the plant to extract water fro:' 

 these several soils, and by difference, the quantity of w;> 

 ter in them that was unavailable to the tobacco plant. 



How do the Roots take Hyi^roscopic Water from the 

 Soil ? — The entire plant, when living, is itself extremely 

 hygroscopic. Even the dead plant letains a certain pro- 

 portion of water with great obstinacy. Thus wheat, 

 maize, starch, straw, and most air-dry vegetable substances, 

 contain 12 to 15" |^ of water ; and when these matters are 

 exposed to damp air, they can take up much more. Ac- 

 cording to Trommer {Bodenkwide., p. 270), 100 parts of 

 the following matters, when dry, absorb from moist air iu 



12 24 4S 72 



As already explained, a body is hygroscopic because 

 there is attraction between its particles and the particles 

 of water. The form of attraction exerted tlius among 

 different kinds of matter is termed adhesive attraction, or 

 simply adhesion. 



Adhesion acts only through a small distance, but its in- 

 tensity varies greatly within this distance. If we attempt 

 to remove hygroscopic water from starch or any similar 

 body by drying at 212°, we shall find that the greater 

 part of the moisture is easily expelled in a short time, 

 but we shall also notice that it requires a relatively much 

 longer time to expel the last portions. A general law of 

 attraction is that its force diminishes as the distance bo- 

 tweeu the attracting bodies increases. This has been ex- 



