484 VENTILATING SYSTEM 



daughter-cells give rise to a colourless complementary tissue which 

 fills up the air-chamber. A curved cell-layer, convex towards the 

 inside, undergoes tangential divisions and becomes the " lenticellar 

 meristem," which continually cuts off additional complementary cells on 

 its outer side. The pressure of the steadily expanding complementary 

 tissue causes the epidermis to bulge outwards, and finally to burst; 

 the complementary tissue then protrudes in places, hence the rough 

 surface of the lenticel. On older branches, which are already covered 

 with periderm, the lenticels arise from the phellogen, which, at certain 

 points, produces complementary tissue with abundant air-spaces in 

 place of the normal uninterrupted layers of cork, and thus becomes 

 locally converted into lenticellar meristem. The layer of cork above 

 a developing lenticel suffers the same fate as the epidermis in the 

 other type of development, being distended and finally ruptured by the 

 growing complementary tissue. 



