CORK 67 



Externally there may be developed secretory structures 

 such as the nectaries of flowers, etc. 



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 

 them impermeable to water. 

 The cells die shortly after sub- 

 erization occurs and remain FIG. 36. Cork (i), 



. (2) layers m bark. 



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- 



