LENTICELS 



139 



lenticels, whose development often commences slightly before 

 that of the cork. 



The first lenticels usually arise beneath the stomata of the 

 young stem, where strips of cambium are formed in the sub- 

 epidermal layer by the customary tangential divisions. These 

 cambial strips divide 

 very actively, cutting 

 off segments on both 

 sides. Those on the 

 inner give rise to radial 

 rows of phelloderm, 

 whilst those on the 

 outer remain thin- 

 walled and unsuberised, 

 but sooner or later round 

 off and lose all connec- 

 tion with one another 

 (Fig. 68, I.). This loose 

 tissue 1 is formed in con- 

 siderable bulk, so that 

 it leads to a gradual 

 elevation and ultimate 

 rupture of the overlying 

 epidermis. The tissue 

 of the lenticel is thus 

 exposed, and air from 

 the surrounding atmo- 

 sphere can freely circu- 

 late between its cells 

 and, by way of the 

 narrow air-spaces be- 

 tween the cells of the 

 phellogen and phello- 

 derm, into the inter- 

 cellular spaces of the cortex. The rounding-off of the constituent 

 cells and consequent development of the intercellular spaces 

 vary considerably, so that the lenticel may be spongy («.,§'. Elm, 

 Birch) or relatively compact {e.g. Willow, Spindle-tree). The 

 t Forming the so-called " complementary tissue." 



Fig. 67. — Transverse section through the 

 deep-seated cork in the stem of the 

 Barberry (Berberis) showing two layers 

 of cork-cells (Co.) situated between 

 the cork-cambium (C.C.) and the zone 

 of sclerenchyma (Sc). 



