ROOT HAIRS 



273 



which it passes. Eventually the lateral emerges from the root of which it 

 forms a branch, and becomes an externally visible part of the root system. 



Root Hairs. — These structures are confined to the root hair zone, which 

 may be from a few millimeters to many centimeters in length, depending on 

 the species and the conditions under which the root develops. Since the root 

 hair zone lies back of the region of cell enlargement, there is no fonvard 

 progression through the soil of the individual root hairs along the axis of the 

 root. In any rapidly growing root tip new hairs are continually developing 

 just back of the zone of elongation. New root hairs are thus constantly de- 



FiG. 71. Three stages in the formation of a lateral root. Meristematic cells 

 (stippled) arise in the pericycle and the cells of the lateral root develop from these. 

 Redrawn from Holman and Robbins (1938) after van Tieghem. 



veloping in contact with different portions of the soil, a fact which is of 

 fundamental significance in the absorption of water and mineral salts. Root 

 hairs are often short-lived structures, and frequently die within a few weeks 

 or even less after they develop. In some species (honey locust, redbud, some 

 composites) the walls of root hairs become lignified and they may persist for 

 a year or longer. 



Very little information is available regarding the abundance and distribu- 

 tion of root hairs on the root systems of mature plants. Dittmer (i937) 

 found root hairs to be present on all the roots of a four months old rye plant. 

 This plant bore a total of about fourteen billion root hairs. On the other 

 hand some species, especially certain conifers, ordinarily bear no root hairs. 

 The roots of some species, such as maize, produce abundant root hairs when 

 they develop in the soil or in moist air, but few or none when they develop 

 under water as in a solution culture. The roots of many species, on the other 



