142 



BOTANY 



PAET I 



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. 150, 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. 



In most woody plants, particularly in Dicotyledons, cortical pores, 

 or lenticels (Fig. 156), 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 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 complementary cells (I) 



■$0 



doer 



M- P l 



Fig. 156.- 



Transverse section of a lenticel of Saw,biwu* nigra, e, Epidermis ; pli, phellogen ; I, com- 

 plementary cells ; pi, phellogen of the lenticel ; pd, phelloderm. ( x 90.) 



traversed by intercellular spaces. On the inside, however, a phello- 

 derm 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, the intermediate bands 

 or closing layers are developed from the phellogen alternately with 

 denser layers of cells, which, as in the case of the epidermis, become 

 eventually ruptured. 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 develop- 

 ment at a corresponding depth. The lenticels are .so constructed, 

 in Primus avium and Behda, that they can accommodate them- 

 selves to growth-in-thickness ; in Quercus Suber, Fraxinus Ornus, they 

 are not in a condition to do so ; while in Bhamnus Frangula and Pirus 

 Malus each lenticel gives rise to a group of lenticels. The develop- 



