THE ROOT HAIRS 



91 



Fig. 42. — Cross section of a root in the region of 

 the root hairs. 



without being shoved and jammed forward as would be the case 

 if elongation were kept up in the remoter parts. 



Under average conditions the water in the soil exists as a 

 thin film around each soil particle and holds in dilute solution 

 (o.oi per cent, to 0.03 per 

 cent.) some of the con- 

 stituents of the particles 

 that are necessary to 

 plants. To get the water 

 and solutes it is obviously 

 necessary that many fine 

 outgrowths from .the root 

 should reach out on all 

 sides, and, pressing them- 

 selves against the soil 

 particles, become immersed in the films of water. 



The Root Hairs. — The root elongates close to its apex, and 

 2 or 3 mm. back from this it ceases to grow in length, and some 

 of the epidermal cells here grow out in the form of slender tubes 

 known as root hairs (Fig. 42). So far as recorded . measure- 

 ments show these may become from a fraction 

 of a millimeter to 8 millimeters in length. 



Growing only at its point a root hair reaches 

 out through the humid atmosphere of the soil 

 interspaces until it strikes a solid particle, 

 when it bends about this and flattens out over 

 it to a certain extent (Fig. 43). At the place 

 of contact the delicate cellulose wall of the 

 hair becomes somewhat mucilaginous and is 

 thus all the better able to cling on and imbibe 

 water, and when it reaches a soil particle and 

 becomes fastened to it this contact seems to 

 act as a stimulus to stop its further growth in length. By 

 means of the root hairs the roots are able not only to make close 

 contact with soil and soil water, but they also increase their 

 absorbing surface many times — from five to twelve times ac- 



FiG. 43. — Apex of 

 root hair flattened out 

 over and imbedding 

 soil particles. 



