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 phlcem. 



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: 



1. Cork 



2. Cork cambium (phellogen) 



3. Secondary cortex 



4. Protophloem 



5. Secondary phloem 

 6. Cambium 



7. Secondary xylem 



43 



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 Calif ornicum) 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. 



