706 ECOLOGY 



derm together are known as periderm. Eventually this phellogen 

 layer ceases its activity and a new phellogen layer develops under- 

 neath the old, though in some cases the original periderm layer 

 persists for years (as in the silver fir), or even throughout life (as in 

 the beech), the lateral growth of the phellogen keeping pace with 

 the annual increase of the secondary wood. 



The phellogen consists of delicate plasmatic tabular cells, which di- 

 vide tangentially, the outer half becoming cork, while the inner remains 

 phellogen and divides again. The cork cells also are tabular, and, 

 as there is little displacement or radial division, they are arranged in 

 radial rows. The walls soon become partly or wholly suberized, that 

 is, a complex, fatty substance known as suberin replaces or is added to 

 the original cellulose after which the cells die. The walls of mature cork 

 cells are brownish, the lumina containing air, and sometimes tannins, 

 crystals, and other excretions; intercellular air spaces are practically 

 wanting. 



In the cork oak, which supplies commercial cork, the periderm layers are very 

 thick. In certain trees (as in the winged elm, sweet gum, and hackberry) cork 

 develops irregularly, forming peculiar warts, wings, or thorns. Often there are 

 annual growth rings in cork tissues comparable to those in wood, broad layers of 

 large cells alternating with thin layers of narrow cells. In the paper birch and 

 in various other trees and shrubs, the bark is exfoliated in thin sheets or strips by 

 reason of an alternation of dense and loose layers of this sort (fig. 1034). 



The influence of external factors upon cork development. It has been 

 shown elsewhere that under water, phellogen develops into aerenchyma 

 rather than into cork, and that the development of cork layers beneath 

 lenticels is favored by desiccation. Probably cork formation is favored 

 generally by desiccation resulting from transpiration, the cells originating 

 from the phellogen tending to deposit suberin, just as epidermal cells 

 deposit cutin or surface wax under similar conditions. This view is 

 supported by the fact that on the inner side, where less exposed to 

 desiccation, phellogen cells develop into phelloderm, which is a loose 

 tissue resembling the complementary tissue of lenticels and the products 

 of phellogen when developed under water. In the sassafras and proba- 

 bly in trees generally, cork formation is stronger on the lighted than on 

 the shaded side of the branches. 



Possibly the strong radial growth in developing phellogen is due to the fact that 

 the path of least resistance is in the radial direction. When a branch is cut, the liv- 

 ing cells bordering the wound are incited to active growth, and the wound is covered 

 by a so-called callus, one of the most prominent elements of which is cork; such cork, 



