154 PART II. ANATOMY AXD HISTOLOGY. [ 35 



organs corresponding to the stomata of the epidermis, serving, like 

 them, to admit air to the living internal tissues ; these are the 

 Lenticels. They are usually circumscribed circular areas of the 

 periderm where the cork-cells formed in the course of the summer 

 are not arranged closely together, but are separated by intercellular 

 spaces. In winter the lenticels are closed by a layer of ordinary 

 periderm. They are most easily detected in branches of one year's 

 growth, where they are to be seen in the summer in the form of 

 brownish or whitish specks. When the periderm of the stem is 

 superficial, the lenticels are developed under the places where the 

 stomata occur in the epidermis, and these spots are commonly the 

 starting-points of the formation of the periderm; but this is not the 

 case in stems with a deep periderm, nor is it ever the case in roots. 



In many trees, as the Birch, 

 the lenticels become much 

 extended in width by the 

 growth of the branch in 

 circumference. When the 

 periderm is very thick, as 

 in the Cork-Oak, the len- 

 ticels form deep canals 

 filled with a pulverulent 



FIG. 120.-Lenticel in the transverse section of a aSS of cells. Sometimes 



twig of Eider (x 300): c epidermis; q pheiiogen; i lenticels are not formed ; 



cells, and pi the pheiiogen of the lenticel ; Ic cortical , , , 



parenchyma containing chlorophyll. the y are not P reSent m 



the stems of some plants 



which have a pericyclic pheiiogen (e.g. Vitis, Clematis, Rubus, 

 Lonicera). 



The phcllodcrm or secondary cortical tissue, the tissue formed 

 internally from the pheiiogen, consists of cells which have 

 essentially the same structure as those of the primary cortex : the 

 secondary cortex can, however, be distinguished from the primary 

 by the regular radial rows in which, like those of the periderm, its 

 cells are arranged. The cells have protoplasmic cell-contents, and, 

 when developed near the surface of aerial stems, they contain 

 chloroplastids ; their walls are usually thin and consist of cellulose, 

 but, like those of the cells of the primary cortex, they may become 

 more or less thickened and eventually lignified. 



Just as the periderm replaces the disorganised epidermis as a 

 tegumentary tissue, sa the phelloderrn replaces the primary cortex 

 as a nutritive (metabolic) tissue when the primary cortex becomes 

 obliterated under the conditions explained on p. 149. 



