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THE ROOT 165 
HistoLocy oF Monocotyt Roor 
In this connection we will discuss only the type of greatest 
pharmacognic importance, #.e., the mesophytic type as seen in its 
most typical form in the transverse section of a Smilax root. 
Examining such a section from periphery toward the center, 
one notes the following: 
1. Epidermis (derived from dermatogen) of a single layer of 
cells many of which give rise to root-hairs. 
2. Hypodermis (derived from periblem) of one, two or three 
layers of cells whose walls are extremely thickened. 
3. Cortex (derived from periblem) consisting of a broad zone 
of parenchyma cells many of which contain starch grains. 
4. Endodermis (derived from periblem) of one layer of endo- 
dermal cells whose walls are extremely thickened through the 
infiltration of suberin and lignin. 
5. Pericycle, also called pericambium (derived from periblem), 
of one or more layers of meristematic cells whose walls are 
extremely thin. 
6. A radial fibro-vascular bundle (derived from plerome) of many 
alternating xylem and phloem strands and hence polyarch. The 
phloem tissue consists of phloem cells and sieve tubes arranged in 
small ovate areas beneath the pericycle and between the outer 
reaches of the xylem strands. The xylem is composed of xylem 
cells, tracheze and wood fibers. 
7. Medulla or pith (derived from plerome) composed of paren- 
chyma cells containing starch and often showing xylem patches 
cut off and enclosed within it. 
The tissues within the endodermis collectively constitute the 
STELE. 
The Smilax root resembles most monocotyl roots in show- 
ing no secondary increase in thickness which is sometimes 
called secondary growth. Secondary increase in thickness is the rule 
in roots of most dicotyledons and gymnosperms and is due to the 
formation in those roots of acambium. No such tissue arises in 
most monocotyl roots. However, some difference in diameter 
will often be noted between sections cut through the same root at 
different levels above the root cap, those cut nearer the stem end 
