SECONDARY MERISTEMS 331 



woody stem is carried on by the activity of a secondary 

 (vascular) meristem (cambium) which arises in the un- 

 differentiated tissue between the xylem and the phloem 

 of each primary bundle, and extends across the rays 

 to form a continuous ring x of meristematic (actively 

 dividing) tissue which runs round the stem. By far the 

 greatest part of the mass of a tree-trunk is formed of 

 secondary xylem, which is the wood of the trunk, by the 

 activity of the cambium. 



Another secondary meristem, the cork cambium or 

 phellogen 2 , arises in the stems and roots of most woody 

 plants, external to the vascular tissues. In stems it 

 arises most often in the outermost layer of the cortex, 

 immediately below the epidermis, but sometimes in the 

 epidermis itself, in the pericycle, or in a deep layer of the 

 cortex. This forms the tissue (cork) of which the outer 

 bark of a tree or shrub is largely or wholly composed. 



The Cambium and Secondary Thickening. The cam- 

 bium consists of active meristematic cells, typically 

 elongated in the direction of the axis of the stem, and 

 also tangentially, while they are often narrow in the 

 radial direction. The top and bottom walls of each 

 cell are commonly inclined like a roof or penthouse, 

 the slopes being directed tangentially (Fig. 54, A). 

 Properly speaking, the term cambium is confined to a 

 single layer of cells, which continually divide by new 

 tangential walls (Fig. 54, A-C), one of the daughter cells 

 at each division becoming a tissue mother cell, giving 

 rise to vascular, fibrous or parenchymatous tissue 

 elements of the secondary xylem or phloem, according 

 to the side on which it is cut off, the other growing again 

 to form the cambial cell. The term is often, however, 



1 As seen in cross-section : it is really, of course, a cylinder. 

 * Greek ^eAAog, cork, and yevvdw, produce. 



