DIV. 



MORPHOLOGY 



59 



contents and may be thin- or thick-walled, arise in the same way and are called 

 PHELLOID TISSUE. The BARK, which is met with on still older stems as the 

 limiting tissue, consists of tissues of still more varied structure (cf. p. 163). 



LENTICELS. The formation of a covering of cork without inter- 

 cellular spaces in place of the epidermis would prevent gaseous 



Fio. 58. Transverse section of 

 bottle-cork, (x 120.) 



Fio. 59. Transverse section of the cork layer 

 of a Lime twig. The cell walls are left 

 white, while the dead contents are dotted, 

 (x 120.) 



exchange between the interior of the stem and the atmosphere were 

 the stomata not replaced in some way. This is effected in some 

 plants (e.g. species of Clematis, Pitis, Lonicera) by porous cork, in 



P d pi 



FIG. 60. Transverse section of a lenticel of Sambucus nigra. e, Epidermis ; ph, phellogen ; 

 I, complementary cells ; pi, phellogen of the lenticel ; pd, phellodenn. (x 90. After STBASBUROER. ) 



which small circumscribed oval or circular areas consist of somewhat 

 smaller suberised cells with intercellular spaces between them. Usually, 

 however, lenticels are present, rough porous warts elongated or 

 spindle-shaped in outline which are readily seen by the naked eye 

 on the cork of most trees. They consist of dead unsuberised tissue 

 rich in intercellular spaces (COMPLEMENTARY TISSUE) interrupting 



