Page:The Oak.djvu/133

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115
THE TREE—ITS SHOOT-SYSTEM.

cell-walls do not become completely suberized 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 protrude, 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 suberized 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 following spring new layers of loose, swelling packing-cells