CELLULAR ANATOMY OF THE ROOT TIP 



145 



regio'ns 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 be- 

 hind. The growth zone passes gradually 

 into the absorptive zone where the fol- 

 lowing tissues become quite well denned: 

 (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- FIG. 129. The radi- 

 ' ductive tissues forming a central cylinder, c l e of a Pea seed l m g 



known as the vascular cylinder. marked , t( ? show the r A e ; 



Tl . , , ,, ' ,, . , ,. gions of elongation. At 



It is to be noted that the epidermis of the left> radic i e just after 



roots, unlike that of leaves and stems, being divided into spaces 



has no cutinized walls and contains no of about of an inciTin 



stomata or other openings for the entrance width. At the right, 



of air, although so many active cells re- radicle several ^ hours 



, ,. ... TT after marking, showing 



quire much oxygen for respiration. How- the region where elonga _ 



ever, openings are not necessary, for the tion is ta king place, 

 uncutinized walls offer practically no re- Only the marks near the 

 sistance to the passage of water, which tips have spread apart, 

 usually carries in solution oxygen enough After Ha y den - 

 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 ones form ahead, and in this 



