126 
PHARMACEUTICAL BOTANY 
2. Hypodermis. 
3. Primary cortex with usually small intercellular spaces. 
4. Endodermis, or innermost layer of cells of the cortex with 
lenticularly thickened radial walls. 
5. Pericambium of one to two layers 
of actively growing cells which may 
produce side rootlets. 
6. Radial fibro-vascular bundle of 
four, rarely two or three or five or six 
phloem patches alternating with as 
many xylem arms. Not uncommon to 
find bast or phloem fiber along outer 
face of each phloem patch. Xylem 
has spiral tracheae, internal to these a 
few pitted vessels, then, as root ages, 
more pitted vessels, also xylem cells 
and wood fibers make their appearance. 
7. Pith, a small zone of parenchyma 
cells. 
B. Of Secondary Growth (Most official 
roots). 
At about six weeks one notes cells 
dividing by tangential walls in the inner 
curve of phloem patches. This is in- 
trafascicular cambium. A single layer 
of flattened cells starts to cut off on 
its inner side a quantity of secondary 
xylem and pushes out the patches of 
bast fibers, adds a little secondary 
phloem on the outer side. Secondary 
xylem finally fills up the patches between 
the arms. The patches of bast fibers get flattened out. The 
pericambium has a tendency to start division into an inner and 
outer layer. The outer layer becomes a cork cambium (phellogen) 
surrounding the bundle inside of the endodermis. It cuts off cork 
tissue on its outer face, hence all liquid material is prevented from 
filtering through and cortex including endodermis, as well as the 
Pig. 60.—Cross-section of a 
young root of Phaseolus muJti- 
florus. A, pr, cortex; m, pith; 
x, stele or central cylinder—all 
tissue within the pericycle, in¬ 
clusive; g, primary xylem bun¬ 
dles; b, primary phloem bun¬ 
dles. B, cross-section of older 
portion of root; lettered as in A ; 
b', secondary phloem, k, cork. 
(Stevens, after Vines.) 
