88 THE ROOT 



water, except for a short distance from the tip backward. 

 Only the parts most recently formed are active in absorption. 



53. The production of new rootlets is thus of high importance. 

 Accordingly, as long as the plant grows above ground, and expands 

 fresh foliage from which moisture largely escapes into the air, so long 

 it continues to extend and multiply its roots in the soil beneath, re- 

 newing and increasing the fresh surface for absorbing moisture in 

 proportion to the demand from above ; and when growth ceases above 

 ground, and the leaves die and fall or no longer act, then the roots 

 generally stop growing, and their soft and tender tips harden. From 

 this period, therefore, until growth begins anew the next spring,' is the 

 best time for transplanting, especially for trees and shrubs. 



54. The action of root hairs. It has already been 

 noted in the laboratory that the tip of the seedling root is 

 for a space smooth, but that at a little distance back a 

 thick covering of root hairs soon arises. These not only 

 insinuate themselves into the interspaces of the soil along- 

 side of the root, and suck up whatever water may be 



there ; but they apply 

 themselves closely to the 

 soil particles, the walls 

 even becoming lobed and 

 distorted in order to gain 

 closer contact with the 



27. A root hair, much magnified. It is . , 



seen to be a tubular outgrowth uneven particles compos- 



from an exterior cell of the root, j n p. ^he Soil (Tiff. 27). 

 in this case much distorted. ^ 11 



tor adhering to the sur- 

 faces of the latter are certain substances much needed by 

 the plant. These substances, mineral salts, 1 are not re- 

 moved by the simple flow of soil water, 2 but remain firmly 

 bound until acted upon by the root hairs. At the points of 

 contact, the root hairs excrete an acid which acts to release 



1 Salts such as potassium nitrate (saltpeter), magnesium sulphate, 

 calcium phosphate, etc. 



2 Fertilizers applied to land and dissolved by the rain are held in the 

 same manner by the soil, until taken by the roots of the crops. But if 

 applied when the ground is frozen, the fertilizers do not penetrate the 

 absorbent soil to the same extent, and much is washed away by surface 

 drainage, and lost. 



