478 



VENTILATING SYSTEM 



meristematic zone, which is continuous with the phellogen on every 

 side : but the hulk of the lenticel is made up of so-called complementary- 

 tissue containing a large number of intercellular spaces. Stahl dis- 

 tinguishes between two leading types of lenticel, which differ in the 

 nature of their complementary tissue. In the simpler of these types 

 (illustrated by Sambucus [Fig. 1 90], Lonicera, Euonymus, Salix, 



Fn;. 100. 

 Lenticel of Sambucus nigra in transverse section (summer of the second year of growth). 



Ginkgo, etc.), the complementary cells are somewhat closely united, 

 and form a tissue which is relatively compact and firm, in spite of the 

 numerous air-spaces that it contains. The second type, which is much 

 more widely distributed, is characterised by the rounded form and 

 loose arrangement of the complementary cells. This incoherent tissue, 

 which in addition is usually brittle or powdery, is prevented from 

 falling to pieces by the circumstance that the meristematic zone of 

 the lenticel occasionally produces so-called closing layers, composed of 

 one or more series of cells which are firmly united to one another. 

 These closing layers constitute relatively firm diaphragms, which give 

 cohesion to the whole complementary tissue ; they are traversed by 

 radial intercellular passages, and hence do not interfere with gaseous 

 interchange. Sooner or later the closing layers burst, owing to the con- 

 tinued production of complementary tissue, and are replaced by freshly 

 formed layers of the same kind. Examples of the second type of 

 lenticel are furnished by Ulmus, Robinia, Sojihora, Alnus, Betula, 

 Sorbus, Primus, Aesculus, etc., etc. Both kinds agree in the fact that 

 the complementary cells are dead, more or less full of air, and pro- 

 vided with cell- walls which are thin and frequently brown in colour. 



