108 HISTOLOGY OF ANGIOSPERM STEM, BOOT, ETC. 



most layer of the cortex, just below the epidermis. Each 

 of these hypodermal collenchyma-cells first elongates in 

 the radial direction, and divides by a tangential wall into 

 an outer and an inner cell ; the latter divides again in the 

 same way, then successive tangential divisions, accompanied 

 by radial growth, occur in the middle (phellogen) cell, 

 giving rise externally to a row of cork-cells. Thus we 

 get a radial row of cells the outermost represents the 

 outer half and the innermost the inner half of the original 

 collenchyma-cell, the lowest cell but one being the phello- 

 gen-cell. At a later stage the phellogen cuts off cells on 

 its inner side these retain their protoplasm, contain chloro- 

 plasts, and add to the cortex, forming the " secondary 

 cortex " or phelloderm. 



123. The Development of Lenticels (Fig. 32) can 

 also be studied in Elder. On the young stem the lenticels 



[PIDERMIS LOOSE CELLS 



PHEULOGEN CORTEX 



Fig. 32. Section through a Lenticel. Phellogen = Cork-cambium. 



appear as projections, each forming a groove with raised 

 lips, and on examining the younger parts of the twig the 

 incipient lenticels appear as light-brown spots on the 

 otherwise green surface. Transverse sections taken at 

 these spots may show that just below a.stoma divisions 

 occur in the hypodermis, giving rise to a meniscus-like 

 layer of phellogen, which produces on its inner side a 

 little phelloderm and on its outer side rows of loose brown 

 " packing tissue." Then the epidermis becomes torn to 

 form the fissure-like lenticel. The development of the 



