PLANT ORGANS AND ORGANISMS 
91 
Thus, in a transverse section made through a portion of a Dicotyl 
root showing secondary growth , the following regions are noted pass- 
ing from periphery to center: 
1. Cork 
2. Cork cambium (phellogen) 
3. Secondary cortex 
r 4. Protophloem 
5. Secondary phloem 
6. Cambium 
7. Secondary xylem 
8. Protoxylem 
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Strands of cells extending radially 
from the cortex to the center of the 
section separating each open fibro- 
vascular bundle from its neighbors. 
These are called medullary rays. 
Histology and Development of a 
Dicotyl Root (California Privet). — A. 
Make a permanent mount of a T. S. 
of the root of the California Privet 
( Ligustrum Calif or nicum) cut just 
above the root cap, and note the fol- 
lowing structures, passing from pe- 
riphery toward the center: 
1. Epidermis, composed of a layer 
of epidermal cells whose outer walls 
have been infiltrated with a substance 
called Cutin. 
Fig. 37. — Cross-section of a 
young root of Phaseolus mulli- 
Jlorus. 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.) 
2. Hypodermis, a layer of somewhat thick walled cells just be- 
neath the epidermis. 
3. Cortex, contposed of cortical parenchyme cells with small 
angular intercellular air spaces. 
4. Endodermis, or innermost layer of cells of the cortex, whose 
radial walls are lenticularly thickened. 
5. Pericambium, of a layer of actively growing meristematic 
cells, which has the power of producing lateral rootlets. 
