312 



PHYSIOLOGY 



tern, however, only the younger parts are permeable to water, since with 

 age the surface cells become altered, or usually are underlaid and finally 



replaced by corky or cutinized 

 tissues, whose walls are nearly 

 waterproof. But as the roots are 

 growing at the tips and branching, 

 there are always young and per- 

 meable parts. 



Root hairs. — The surface cells 

 of the young root in most land 

 plants, at a short distance behind 

 the growing apex, branch, sending 

 out tubular extensions, the root 

 hairs (fig. 625), which push their 

 way among the soil particles, dis- 

 placing some and being deformed 

 by crowding against others, to 

 which they often adhere strongly. 

 These root hairs increase 5-12-fold 

 the permeable area of the root, and 

 by their size and radial position 

 come into immediate contact with 



— Root hairs of lettuce, with 

 adherent soil grains (s). — From Part TTT. 



a cylinder of soil 3-8 mm. in di 

 ameter (fig. 626). They anchor the young root 

 thoroughly, since they adhere so firmly to the soil 

 particles that they tear away from the root when 

 that is pulled out of even the loosest soil; and if 

 by chance they come away, they bring with them 

 the adherent grains. The root hairs are transient, 

 not living through even one growing season. They 

 die away on the older parts of the roots, from 

 which the hair-bearing cells usually slough off; 

 but new hairs are being formed continually, during 

 the growth of the root in length, just behind the 

 advancing apex. (See Part III, p. 495, for varia- 

 tion of root hairs and for kindred topics.) 



Soil. — The soil, into wdiich roots clothed with 

 root hairs spread, consists of particles of weathered 

 or comminuted rock of various kinds, usually 



Fig. 626. — Seedling 

 of mustard ; a, grown 

 in moist air ; b, grown 

 in sand and withdrawn 

 to show mass of soil 

 grains clinging to root 

 hairs. — After S.ACIIS. 



