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 pericycie. 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 2 
protective tissue is developed by the eee 
layers of cells beneath. Usually the HEL 
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 
DIITy 
NIVT TD 
3 7. A BE 
Fig. 131.— Diagram of a 
lengthwise section through the 
region of the root back of the 
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 
hair zone, showing the changes 
in the epidermis and cortex. e, 
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 
a secondary cortex which it also en- ° tinder. Atghiy magained. 
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 
