CORK 67 
Externally there may be developed secretory structures 
such as the nectaries of flowers, ete. 
99. Cork. At first the cutinized external wall of the 
epidermis of the stem serves to prevent excessive water 
loss. When the stem enlarges the increased circumfer- 
ence is met by the enlargement or multiplication of the 
epidermal cells. There is a limit, however, for most 
stems to this epidermal growth and furthermore as the 
stem becomes enlarged the one layer of cells is no longer 
sufficient protection against water loss and especially 
against mechanical injury. There is accordingly formed 
beneath the epidermis a layer of meristem cells called 
phellogen or cork cambium, which gives rise (by periclinal 
divisions) to radial rows of cells without intercellular 
spaces, whose walls become strongly suberized by the de- 
position within them of a 
fatty substance or substances 
called suberin, which makes o 
them impermeable to water. [Loar ae 
The cells die shortly after sub- 
erization occurs and remain Fic. wig @), subepidermal, 
filled with the broken-down ; 
protoplasm or become filled with air. These layers of 
cork cells, owing to the suberization, cut off the passage 
of water toward the exterior and the epidermal cells 
accordingly die. With the growth of the stem in 
circumference these are soon ruptured here and there 
and gradually peel off. Since the outer cork cells are 
also dead they cannot enlarge and so as the stem 
grows longitudinal fissures occur in the cork extending 
down nearly to the living phellogen, which however 
being alive is able to increase in circumference and 
thus keep pace with the increasing circumference 
of the stem. Sometimes this phellogen layer is per- 
ni 
