ANATOMY OF THE OLDER PORTIONS OF THE ROOT 147 



rangement of xylem and phloem shows adaptation, in that it per- 

 mits the absorbed substances to reach the xylem without passing 

 through the phloem. The vascular cylinder is bordered by a 

 chain of cells, known as the pericycle. The pericycle joins the 

 endodermis or starch sheath which is the chain of cells forming 

 the innermost layer of the cortex. Aside from the fact that 

 branches or lateral roots develop from the pericycle, the functions 

 of the endodermis and pericycle in roots are not well understood. 



Anatomy of the Older Portions of the Root. Not far back of 

 the hair zone, as indicated by the brownish color and the slough- 

 ing off of the epidermis with its dead root hairs, there appear 

 some anatomical changes, such as the formation of a corky cover- 

 ing, enlargement of conductive and strengthening tissues, and 

 the development of branches. As this region of the root becomes 

 older, these anatomical changes become more prominent. 



Since the epidermis behind the hair zone dies and falls away, 

 absorption is limited to the tip region of the root. Accompany- 

 ing the death of the epidermis, a 

 protective tissue is developed by the 

 layers of cells beneath. Usually the 

 cells just beneath become cutinized 

 and take the place of the epidermis 

 as a covering. In Grass roots the 

 layers of cells just beneath the epi- 

 dermis thicken their walls, thereby 

 forming over the root a hard woody 

 rind similar to that of Grass stems. 

 Commonly in roots there is also 

 formed in the region of the pericycle 

 a meristematic band of cells, known 

 as the cork cambium, which by divid- 

 ing parallel to the surface of the root 

 adds layers of cork on its outer side 

 and cortex cells on its inner side, thus 

 forming a protective covering and 

 a secondary cortex which it also en- 

 larges as the root grows older. Since cork is impervious to water 

 and the foods contained, by the formation of cork in the region 

 of the pericycle, the first or primary cortex has its conductive 

 connections with the vascular bundles cut off and its death 



FIG. 131. Diagram of a 

 lengthwise section through the 

 region of the root back of the 

 hair zone, showing the changes 

 in the epidermis and cortex. e t 

 epidermis dead and sloughing 

 off; k, cork cambium on the in- 

 ner side with cork and dead cor- 

 tex between it and the epidermis; 

 c, secondary cortex; v, vascular 

 cylinder. Highly magnified. 



