CELLULAR ANATOMY OF THE ROOT TIP 



145 



regions of the root increase in diameter, but almost all elongation 

 takes place in the growth zone, as shown in Figure 129. The 

 meristematic zone is thus so situated that the new cells formed 

 by it may be added both to the root cap, the thickness of which 

 is thereby maintained in spite of its being rapidly worn away on 

 its outer surface, and to the growth zone, the older portions of 

 which are constantly taking on the fea- 

 tures of the absorptive zone just behind. 

 The growth zone merges imperceptibly 

 into the absorptive zone where the fol- 

 lowing tissues become quite well defined: 

 (1) a surface layer of cells constituting 

 the epidermis which has most to do with 

 absorption, the special absorptive agents 

 being the root hairs, which, as the section 

 shows, are merely projections of the epi- 

 dermal cells; (2) a broad band of cells 

 just beneath the epidermis and constitut- 

 ing the cortex; and (3) a group of con- 

 ductive tissues forming a central cylinder, 

 known as the vascular cylinder. 



It is to be noted that the epidermis of 

 roots, unlike that of leaves and stems, 

 has no cutinized walls and contains no 

 stomata or other openings for the entrance 

 of air, although so many active cells re- 

 quire much oxygen for respiration. How- 

 ever, openings are not necessary, for the 

 uncutinized walls offer practically no re- 

 sistance to the passage of water, which 

 usually carries in solution oxygen enough 

 to support quite active respiration. 



Through the development of the root hairs the absorptive 

 surface of the root system is much increased, and may be thereby 

 increased from five to six times in Corn, about twelve times in 

 Barley and as much as eighteen times in some other plants. All 

 root hairs are able to absorb regardless of their size, which ranges 

 from a slight bulge near the growth zone of the root to often 

 more than an inch farther back. They live only a few days, 

 but, as they die off behind, new oaes form ahead, and in this 



FIG. 129. The radi- 

 cle of a Corn seedling 

 marked to show the re- 

 gions of elongation. A, 

 radicle just after being 

 divided into spaces of 

 about -aV of an inch in 

 width. B, radicle sev- 

 eral hours after mark- 

 ing, showing the region 

 where elongation is tak- 

 ing place. Modified from 

 Andrews. 



