THE ROOT 
being broader. 
167 
The explanation is that the cells in the broader 
sections have become larger through the process of growth, 
rather than having been augmented in number. 
Histotocy oF Dicotyt Roots 
The typical dicotyl root is a tetrarch one, which is a root 
whose radial bundle exhibits four 
xylem patches alternating with four 
phloem patches. These roots usually 
have an unlimited power of growth. 
The early growth is called primary 
growth which means first growth and is 
represented in sections cut a little above 
the root cap in the region of the root 
hairs. Above the region of primary 
growth is the region of transition where 
changes are taking place in the root 
which lead to the appearance of 
secondary structure. 
A transverse section of a dicotyl 
root in its YOUNG GRowTH shows the 
following structure from periphery 
toward center: 
1. Epidermis (from dermatogen) 
with cutinized outer walls, the cells 
often elongating to form root-hairs. 
2. Primary cortex (from periblem) 
of parenchyma cells with usually small 
intercellular spaces. Whenever the 
outermost layer or layers of cells of 
this region have more thickened walls 
than those beneath, the term hypodermis 
Fic. 100.—Cross-section of 
a young root of Phaseolus multi- 
florus. A, pr, cortex; m, pith; x, 
stele or central cylinder—all 
tissue within the _pericycle, 
inclusive; g, primary xylem 
bundles; 6, primary phloem 
bundles. 8B, cross-section of 
older portion of root; lettered 
as in A; 6’, secondary phloem; 
k, cork. (Stevens, after Vines.) 
is employed for this outer portion of the cortex. 
3. Endodermis (from periblem) or innermost layer of cells of 
the cortex, with lenticularly thickened radial walls. 
4. Pericycle (Pericambium) (from periblem) of one to two layers 
of actively growing cells which may produce side rootlets. 
5. Radial fibro-vascular bundle (from plerome) of four, less fre- 
quently of two, three, five or six primary phloem (protophloem) 
