256 



ELEMENTARY STUDIES IN BOTANY 



which are continually forming beneath. In some plants the 

 root-cap is very easily seen as a conical thickening at the tip 

 of the root ; in others it can be demonstrated only by examin- 

 ing under the microscope longitudinal sections through the 

 root-tip. The presence of such a protective cap in the root 

 is in strong contrast with the stem, whose growing tips are 

 protected by overlapping leaves. 



140. Root-hairs. A short distance behind the root-cap 

 the surface of the root becomes covered by a more or less 



dense growth of hairs, known as 

 root-hairs (Fig. 222). These 

 hairs are outgrowths, sometimes 

 very long ones, from the epi- 

 dermal cells, a single cell' pro- 

 ducing a single root-hair. In 

 fact, the root-hair is only an 

 extended part of the epidermal 

 cell. The root receives water 

 and materials dissolved in it 

 from the soil, and the root- 

 hairs enormously increase the 

 receiving surface. Root-hairs 

 do not last very long; but 

 new hairs are being put out 

 by the elongating root as the 

 old ones behind die, so that 



there is always a zone of active root-hairs near the tip, but 



none on the older parts of the root. 



141. Internal structure. A cross-section of a young root 

 shows two prominent regions (Fig. 223). In the centre is a 

 solid vascular cylinder, often called the central axis. It will 

 be remembered that in the stems of Dicotyledons and Gym- 

 nosperms ( 133, p. 236) the vascular cylinder is hollow, in- 

 closing pith. Investing the solid vascular cylinder of the 

 root is the cortex, which often can be stripped from the 



FIG. 222. Root tips of corn, showing 

 root-hairs and their position in 

 reference to the growing tip. 



