NATURE OF PLANTS 45 



to these particles that it is impossible to detach them without 

 injuring the root hairs (Fig. 29, B). This arrangement admir- 

 ably adapts them to gathering up the water and other crude 

 material in the soil. The tubes are filled with organic acids and 

 other substances which have an affinity for water and such earth 

 substances as the plant requires. In this way the material in 

 the soils are drawn through the delicate walls of the root hairs, 

 a process termed osmosis. The acids in the root hairs also reach 

 through the cell walls and so come in contact with the insoluble 

 substances, dissolving them and thus rendering them also capable 

 of absorption by osmosis. This acid reaction of roots can readily 

 be seen by placing a fresh root upon blue litmus paper which 

 will turn red owing to the acid on the root hairs. The ability of 

 the roots to dissolve mineral matter may be shown by allowing 

 roots to grow over the clean surface of oyster shells or a piece 

 of polished marble buried in the soil. Delicate grooves in the 

 marble show where the roots have dissolved the marble in the 

 course of their growth. It is important to note that not all the 

 space between the soil particles is filled with water because living 

 cells of the root as in the leaf and stem are constantly respiring, 

 absorbing oxygen and giving off COo. The circulation of air 

 in the soil is not only very important for supplying oxygen and 

 bringing about a diffusion of C0 2 , but these gases also materially 

 assist the root in the decomposition of various minerals and in- 

 soluble compounds. 



We are now prepared to understand why the root hairs are 

 developed just back of the zone of elongation. Any disturbance 

 of these delicate absorbing organs, such as the extension of that 

 part of the root to which they are attached, would result in 

 tearing and killing them. 



22. Familiar Facts About Roots. — The character of the root 

 and its relation to the physical properties of the soil explain many 

 familiar phenomena of plant life. We see why plants must be 

 repotted. The root hairs are constantly being formed back of 

 the growing zone and dying off behind. During their short exis- 

 tence they absorb much of the foods in their immediate neighbor- 

 hood, and unless opportunity of extending into new territory is 



