MORPHOLOGY 



149 



of which is due the great resistance of bark to the action of 

 destructive agencies. The peculiar white colour of Birch-bark is 

 caused by the presence of betulin (birch-resin) in the cells. 



In roots which grow in thickness the phellogen usually develops 

 in the pericycle (Fig. 151 B, k), and in consequence of this the 

 primary cortex of the roots dies and peels off. The succeeding 

 phellogen layers are formed in exactly the same way in the root as 

 in the stem. 



Lenticels ( 13f> ). In most woody plants, particularly in Dicotyle- 

 dons, cortical pores, or LENTICELS (Fig. 158), make their appearance 

 simultaneously with the formation of periderm. The lenticels take 

 their origin in a phellogen layer (pi) which, in the case of peripheral 





FIG. 158. Transverse section of a lent.icel of Sambucvs nigra. . Epidermis ; /ih, phellogen ; 

 /, complementary d-lls ; /il, phellogen of the lenticel ; pd, phello<lerni. (x 90.) 



cork formation, almost always develops directly under the stomata. 

 The phellogen, from which the lenticels arise, unlike the cork phellogen, 

 does not form cork cells, but a lenticel tissue composed of COMPLE- 

 MENTARY CELLS (/) with intercellular spaces between them. On the 

 inside, however, a phelloderm is regularly derived from the phellogen. 

 The complementary cells press the epidermis outwards and finally 

 rupture it. Where the complementary cells are only loosely united, 

 intermediate bands or closing layers are developed from the phellogen 

 alternately with the layers of looser tissue ; the closing layers become 

 eventually ruptured. The complementary cells are as a rule not 

 suberised, those of the closing bands are corky and lignified. The 

 cork-forming phellogen joins the phellogen of the lenticels at its 

 margins. In cases where the cork is more deeply seated in the inner 

 tissue, the lenticels begin their development at a corresponding 

 depth from the layer of phellogen. Lenticels serve to maintain the 



