266 



HOW CROPS GBOW. 



the plant, as carefully lifted from the sand in which it 

 grew, and B the same plant, freed from adhering soil 

 by agitating in water. The entire root, save the tip, 

 is thickly beset with hairs. In Fig. 39 a minute portion 

 of a barley-root is shown highly magnified. The hairs 

 are seen to be slender tubes that proceed from, and form 

 part of, the outer cells of the root. 



•The older roots lose their hairs, and suffer a thicken- 

 ing of the outermost layer of cells. These dense-walled 

 and nearly impervious cells cohere together and consti- 

 tute a rind, which is not found in the young and .active 

 roots. 



As to the development of the 

 root-hairs, they are more abund- 

 ant in poor than in good soils, 

 and appear to be most numer- 

 ously produced from roots which 

 have otherwise a dense and un- 

 absorbent surface. The roots of 

 those plants which are destitute 

 of hairs are commonly of consid- 

 erable thickness and remain 

 white and of delicate texture, 

 preserving their absorbent power 

 throughout the whole time that the plant feeds from the 

 soil, as is the case with the onion. 



The Silver Fir [Ahies Picea) has no root-hairs, but its 

 rootlets are covered with a very delicate cuticle highly 

 favorable to absorption. The want of root-hairs is fur- 

 ther compensated by the great number of rootlets which 

 are formed, and which, perishing mostly before they be- 

 come superficially indurated, are continually replaced by 

 new ones during the growing season. (Schacht, Der 

 Bmtm, p. 165.) 



Contact of Roots with the Soil — The root-hairs, as 

 they extend into the soil, are naturally brought into close 



Fig. 39. 



