THE STEM. 



IO9 



the layers of cambium arises nearer the center, the other 

 nearer the periphery of the stem. They are formed from 

 existing cells which resume their power of active growth 

 and division. The development of the tissues from the ex- 

 ternal meristem, or cork cambium, results in the formation 

 of the periderm, while the tissues arising from the internal 

 meristem, or stelar cambium, form the secondary xylem and 

 phloem (fig. 124). 



135. 1. The formation of secondary cortex. — As the 

 cells of the external meristem divide, sometimes the outer 

 segments and sometimes the inner ones 'differentiate into 

 permanent tissues, while the other segment remains as an 

 initial for the next division. Some of the secondary tissue 

 thus produced lies outside of the generating layer, and some 

 inside (fig. 127). The secondary tissues, as a whole, con- 

 stitute the periderm. 



Fig. 



Fig. 126. 



Fig. 125.— A bit of a transverse section of a young stem of Scutellaria splendens at 

 the beginning of the formation of periderm, e, epidermis, some of its cells divided by 

 tangential walls, c, cortex. See fig. 126. Highly magnified. — After Haberlandt. 



Fig. 126. — Same as 125 but older, e, outer half of epidermal cells ; k, cork cells formed 

 by tangential divisions of inner half of epidermal cell (fig. 125) which has become ph, 

 the cork cambium ; c, cortex. Highly magnified.— After Haberlandt. 



136. Periderm. — The tissues formed inside the cambium 

 (phelloderm) are usually similar to the cells of the primary 

 cortex. They form intercellular spaces, and retain their 

 living contents, among which chloroplasts are often present. 

 With the thickening of the outer tissues, however, these 

 usually disappear. 



