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A TEXT-BOOK OF BOTANY 



36. 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. 71). These 



hairs are outgrowths, some- 

 times very long ones, from the 

 superficial cells, a single cell 

 producing a single root-hair. 

 In fact the root-hair is only an 

 extended part of the superfi- 

 cial cell. The root absorbs 

 water and materials dissolved 

 in it from the soil, and the 

 root-hairs enormously increase 

 the absorbing surface. Gen- 

 erally root-hairs do not last 

 very long; but new hairs are 

 being put out by the elonga- 

 ting 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. 



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

 shows two prominent regions (Fig. 73). In the center is a 

 solid vascular cylinder, often called the central axis. It 

 will be remembered that in the stems of Dicotyledons and 

 Gymnosperms ( 24) the vascular cylinder is hollow, en- 

 closing pith. Investing the solid vascular cylinder of the 

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

 central axis like a spongy bark. If the section has passed 

 through the zone of root-hairs, they can be seen coming 

 from the superficial cells. A longitudinal section of a root- 

 tip, in which these regions are very young, is shown in 

 Fig. 70. 



The wood (xylem) and the bast (phloem) of the vascular 



FIG. 71. Root-tips of corn, showing 

 root-hairs and their position in ref- 

 erence to the growing tip: A, grown 

 in soil (higher up the hairs become 

 much more abundant and longer); 

 B, grown in moist air. 



