64 



Irrigation and Drainage 



root in the soil. It has been found that a short way back from the 

 tip end of a growing root, there is at 1 a center of growth, where 

 new cells are developed by repeated enlargements and divisions. 

 On the forward or advancing side of this center the new cells 

 form the root -cap, which in the figure is represented by the cells 



with heavier lines ; while 

 those forming on the rear 

 side of the center are fin- 

 ally transformed into the 

 various structures which 

 constitute the body of the 

 root proper. 



The root- cap is a sort 

 of shield or thimble, under 

 the protection of which the 

 root advances to set aside 

 the soil grains, and the 



^ method of advance is this : 



At the center of growth, 

 new cells are forming and 



Fig. 12. Method by which root-hairs advance enlarging out of the as- 

 through the soil. (Adapted from Sachs.) similated products which 



are being brought down 



from the geeen parts of the plants by osmotic pressure. But 

 when this strong pressure drives the sap into the forming cells, 

 they must enlarge just as the dry wood swells, and in doing so 

 something must give way. As the body of the root is larger than 

 the tip, and as it is already anchored to the soil by the root -hairs 

 and any branches which may have formed, the direction of least 

 resistance is forward, and the cells which are in the interior of 

 the base of the root-cap are crowded forward and the walls of the 

 cap are wedged outward so that the soil grains on all sides are 

 displaced, making room for the end of the root proper to be built 

 into it. The root -cap does not slide forward through the soil, 

 shoving past the soil grains, but its outer and rear cells hold 

 firmly against the earth as the root builds past them, and as fast 

 as they have performed their function they die and new ones are 



?S 



