THE THEE ITS SHOOT-SYSTEM 115 



and their cell-walls do not become completely suberised 

 for a long time, but are capable of swelling : in fact, the 

 rounding off depends on the absorption of water by the 

 cellulose walls and contents. The outer parts of the 

 older lenticel openings are thrown off with the bark- 

 scales, but the inner parts remain, and can be found 

 between the scales in older branches, in the fissures. 



The first points of origin of lenticels are usually 

 beneath the stomata, and the lenticels may be regarded 

 as devices for prolonging the passages of the stomata 

 through the thickening periderm year by year. The 

 cortical cells beneath the stoma become meristematic in 

 effect they continue the phellogen below the stoma, only 

 they divide less regularly and in all directions. The 

 daughter-cells thrown off externally swell up and pro- 

 trude, driving the stomatic cells outwards and apart, and 

 emerging between the ruptured guard-cells as the first 

 packing- tissue. The phellogen or cambium of the 

 lenticel forms phelloderm on its interior in continuation 

 of that formed by the rest of the cork-cambium. The 

 protruding packing-cells dry up eventually, and form the 

 powdery substance seen between the gaping lips of 

 older lenticels. In the autumn the cells formed by the 

 meristem below the packing-cells do not separate, but 

 are suberised and closely and radially arranged like the 

 rest of the cork : in fact, they continue the cork layer 

 as a closing layer beneath the lenticel, thus protecting 

 the tissues beneath through the winter. In the follow- 

 ing spring new layers of loose, swelling packing-cells 



i 2 



