HOT AN V 



of time with the increasing dimensions of the stem. This, however, 

 is the case with the Mistletoe (Viscum album), the number of epidermal 

 cells of which is continually augmented by the formation of new 

 lateral walls, and new cuticular layers are formed where the old 

 cuticle becomes ruptured. On older portions of the stem, however, 

 thickening layers form in single epidermal cells or groups of 

 epidermal cells, and at the same spots similarly thickened cortical 

 cells are developed. The epidermal cells thus cut off die, and 

 the original epidermis is more or less completely replaced on such 



old regions of the stem. In the 

 great majority of plants in which 

 the epidermal layer persists for 

 years it is able to meet the 

 demands made upon it by the 

 growth in thickness of the stem 

 without the assistance of the under- 

 lying cortex. The stems of one of 

 the Maples (Acer striatuw), even 

 when a foot or more in thickness 

 and over forty years old, remain 

 covered with a living, growing, 

 epidermal layer. The striations to 

 which the specific name of this 

 plant refers are caused by a white 

 wax exuding from splits in the 

 outer walls. As a rufe, however, 

 the epidermis on stems which grow 

 , in thickness becomes stretched and 



FIG. 15i>. Diagram of the secondary growth of ., , f^. . . 



the stem and root of Dracoew. ' pc, Central finally ruptured. The tlSSUC of 



cylinder; s, secondary tissue; c, cambium the primary COI'tCX and of the bast 



indicated by a dotte-l line;,, .root. (Adapted fe th expansion am j division of its 



from SCHOI-TK.) , 



cells, can accommodate itself more 



easily than the epidermis to the increased dimensions of the stem, 

 arising from the growth in thickness of the central cylinder. This 

 process of accommodation is particularly noticeable in the primary 

 medullary rays (Fig. 146, pm) between the primary phloem. The 

 formation of the PERIDERM generally begins during the first vegetative 

 period, after the secondary growth has reached a certain stage. The 

 commencement of its formation is indicated by the brown colour of 

 the external surface of the stem, which remains green so long as the 

 epidermis continues alive. The formation of periderm usually occurs 

 at a later period in those plants which have a persisting epidermis ; 

 it is practically wanting only in the species of Mistletoe. The periderm 

 is derived from a secondary meristem, termed the CORK CAMBIUM or 

 I'HKLLOGEN. This phellogen may arise, in the epidermis, in a deeper 

 layer of cells of the primary cortex, or even in the pericycle itself. 



