PLANT ORGANS AND ORGANISMS 
127 
epidermis, shrivel and dry up and separate off at the age of two 
to three months. The cork cambium (phellogen) may lay down 
secondary cortex internal to itself and external to the phloem. 
Patches of cells of the inner layer of pericambium divide rapidly 
and are called interfascicular cambium. These join the intrafasci- 
cular cambium to form a continuous cambium ring which then 
cuts off additional secondary xylem on its inner face and secondary 
phloem on its outer face pushing inward the first-formed or proto- 
xylem and outward the first-formed or protophloem. The medullary 
rays become deepened. 
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: 
<v 
3 
1. Cork 
2. Cork cambium (phellogen) 
3. Secondary cortex 
4. Protophloem 
5. Secondary phloem 
6. Cambium 
7. Secondary xylem 
8. Protoxylem 
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 Californicum) cut just above the root cap, and 
note the following structures, passing from periphery toward the 
center (see Fig. 61): 
1. Epidermis, composed of a layer of epidermal cells whose 
outer walls have been infiltrated with a substance called Cutin. 
2. Hypodermis, a layer of somewhat thick walled cells just be¬ 
neath the epidermis. 
3. Cortex, composed of cortical parenchyme cells with small 
angular intercellular air spaces. 
