THE TREE—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 
12 
