PLANT ORGANS AND ORGANISMS 131 
cambium. This cork cambium has developed secondary cortex on 
its inner face. The cambium has assumed a circular aspect. Just 
beneath the secondary cortex will be found flattened patches of 
protophloem, and beneath these secondary phloem masses have 
Pig. 64.—Photomicrograph of a transverse section of an old portion of 
California Privet root, showing completed secondary development. Note the 
prominent medullary rays (mr); cork ( ck)\ phellogen (ph); secondary cortex 
(between ph and p'); protophloem ( p '); secondary phloem ( p 2 ); cambium (c); 
secondary xylem (* 2 ); tracheae (<); wood fibers ( wf ); and piotoxylem ( x '). 
$ 
been formed through the activity of the cambium. The cambium 
has developed new or secondary xylem on its inner face which has 
pushed the first formed or protoxylem toward the center of the root. 
(Fig. 63.) 
