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XIV. CORK LENTICELS. 



mence to divide, and the segments encroach laterally upon the 

 neighbouring collenchyma cells. Soon is formed under the storha 

 a meniscus-like layer of cells in course of division (Fig. 74, pi), 

 which produce externally colourless cells, which become rounded 

 (/), and internally cortical cells, or phelloderm (pel). The outer 

 cells are distinguished as packing cells. They become brown, 

 but not corky ; and moreover, as they increase in number, they 

 soon cause such a pressure on the epidermis that this is torn into 

 a fissure. In this way is produced the cortical pore, or lenticel. 

 If a twig is examined with the naked eye, the lenticel appears 

 as a groove enclosed between two lip-like swellings ; the brown 

 colour of the packing cells is easily noticeable. On younger 



FIG. 74. Cross-section through a Lenticel of Sa in hue us nif/ra ; e, epidermis; ph, 

 phellogen ; /, picking cells ; pi, cambium of the lenticel ; pd, phellem (x 90). 



parts of the stem the lenticels appear as oval, slightly projecting, 

 spots. Still younger stages are marked out by somewhat brighter 

 colour ; and the section must be taken through such places in 

 order to show the earliest stages of development. Not till after 

 the splitting of the epidermis do divisions begin in the neigh- 

 bouring collenchyma, which result in the formation of the 

 periderm. The packing cells of the lenticel separate from one 

 another ; in proportion as the external cells undergo disorganisa- 

 tion, they are replaced by the action of the cambium. The 

 intercellular spaces of the packing cells are filled with air ; by 

 means of them the inner tissue of the stem communicates with 

 the surrounding atmosphere. They replace, therefore, stomata 



