THE COVERING TISSUES 101 



reference to the stalk of a glandular hair. The gland- 

 ular hairs of cannabis possess a multicellular gland 

 with multiseriate stalk. The glandular hairs of pel- 

 argonium (commonly called geranium) possess a 

 unicellular gland upon a multicellular, uniseriate 

 stalk. Various types of glandular hairs are illustrated 

 in Plate 38. The formation of glandular hairs and 

 the products secreted will be considered under the 

 head of Secreting Tissues. 



PERIDERM 



Periderm occurs on the exposed surfaces of woody 

 roots and stems. The primary epidermal tissues 

 originating from the dermatogen zone do not afford 

 sufficient protection to mature parts of the plant, nor 

 can they keep pace with the growth of the inner tissues. 

 The necessity for a stronger covering arises, and is 

 met by the production of cork by a meristematic tissue 

 located in the primary cortex or immediately beneath 

 the primary epidermis. This meristematic tissue is 

 termed phellogen or bark cambium, and produces cork 

 tissue on its outer surface and phelloderm upon its 

 inner face. With the exception of a region near the 

 phellogen, the cork layers are composed of dead cells. 

 The phellogen tissues are of importance in bark struc- 

 ture and will be considered under that heading in a 

 subsequent chapter. The walls of cork cells are com- 

 posed largely of suberin and the cell contents usually 

 include tannins. The individual cells are always dark- 

 colored, and are so fitted together that very little 

 intercellular space is apparent. Upon surface view 

 the cells are usually polygonal in outline, and occur 

 in thick masses in which the outlines of the individual 



