NUTRITION. 155 



soUes certain substances found therein, notably minute cjuan- 

 titics of ammonia and nitrous acid. ]!y tills means compounds 

 containing nitrogen are constantly being brought to the soil 

 by the rain. 



200. Root absorption. — The structure of the root system 

 has been explained (1| 7S-82). Tlie root hairs come Into 

 close contact with the soil particles, pushing them aside 

 somewhat, and being in turn more or less deformed b)' tlieir 

 resistance (z, s, fig. 171J. So close does the contact of the 

 root hairs and soil grains become that many particles of the 

 soil are imbedded in the walls of the root hairs (fig. 84). 

 The root hairs are not only in contact with the soil particles, 

 but also with the films of water, which occupy the spaces be- 

 tween them (a, fig. 171). They are thus in a position for 

 absorbing water from the adjacent films. 



201. Limit of absorption. — Not only is the water im- 

 mediately in contact «ith the root a source of supply, but 

 even that in the deeper and more distant parts of the soil. 

 For when, by the entrance of some water into the root hair, 

 the thickness of that layer has been decreased, the disturbance 

 of equilibrium causes a flow from neighboring layers to 

 equalize again the surface tensions. This goes on until the 

 films of water upon the soil grains Ijecome so tliin that the 

 water particles are held too tenaciously to be pulled away by 

 the root. There remains in such exhausted soil, which seems 

 dry as dust to the touch, 2 to 12 per cent of water unavailable 

 for the jjlant. 



202. Solvent action. — The root hairs also exert a slightly 

 solvent action upon the soil particles themselves by reason of 

 the carbonic acid and the acid salts which they e.xcrete. By 

 this means various minerals, especially carbonates of lime and 

 magnesia (limestone), which could not be dissolved by the 

 water alone, may be brought into solution. 



Water enters the root hairs by the physical process known 



