128 OUTLINES OF PLANT LIPE. 



tain substances found therein, notably minute quantities 

 of compounds containing nitrogen, which are useful to the 

 plants for food making. 



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

 has been explained (^ 72-76). The root hairs come into 

 close contact with the soil particles, pushing them aside 

 somewhat, and being in turn more or less deformed by their 

 resistance (z, J, fig. 112). So close does the contact of the 

 root hairs and soil grains become that many particles of the 

 soil are embedded in the walls of the root hairs (fig. 51). 

 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 (», fig. 112). They are thus in a position for 

 absorbing water from the adjacent films. 



EXERCISE XXII. • 



To show the location of root hairs and especially their adhesion to soil 

 particles. 



Germinate wheat in sand and when seedlings have several strong roots 

 dig up carefully; shake sharply in water; note where soil clings most 

 tenaciously. Brush away most of this with camelhair brush and examine 

 a bit of this part of root under a low power of microscope. Observe dis- 

 tortion of root hairs, and particles of sand partly embedded in them. 



170. Limit of absorption. — Not only is the water imme- 

 diately in contact with the root a source of supply, but even 

 that in the deep 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 ; and this 

 goes on until the films of water upon the soil grains become 

 so thin 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 plant. 



