MORPHOLOGY 



145 



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cells are replaced from the cells of the ground tissue on the outer side of the 

 ring (p. 128). 



At a later period the cambial ring commences to form cells to the 

 outer side which increase the thickness of the cortex as a thin-walled, 

 parenchy matous ground 

 tissue (Fig. 155). When the 

 cambium has thus become 

 activeon both sides itpresents 

 the appearance of a single 

 initial layer. 



As shown in the diagram 

 (Fig. 156) the growth in 

 thickness of a Monocotyledon 

 differs essentially from that of 

 a Grymnosperm or Dicotyledon 

 represented diagrammatic - 

 ally in Fig. 140. The stem 

 of a Dracaena on which the 

 diagram is based is represented 

 as unbranched. Its primary 

 central cylinder (pc) increases 

 in diameter as the stem grows 

 in length, and thus forms an 

 inverted cone. It is surrounded 

 by the zones of secondary 

 growth, the amount of which 

 diminishes on passing towards 

 the growing point. The 

 secondary growth of the stem 

 is continuous with that of the 

 roots. 



Secondary Growth of 

 Leaves ( 132 ) is always very 

 slight, and is confined to a 

 few Coniferae and Dicotyle- 

 dons with evergreen leaves. 

 The increase in thickness is 

 due to the presence of a 

 fascicular cambium. This 

 produces little or no xylem, 

 the new elements beingmostly 

 added to the phloem. Any 

 elements formed on the side next the xylem are water-conducting, 

 while those adjoining the phloem mostly become sieve-tubes. 



Periderm ( 133 ). It is very seldom that the epidermis, by the 

 division of its own cells, is in a condition to keep pace for any length 



L 



FIG. 155. Transverse section of the stein of: Cordyline 

 (Dracaena) rubra. /', Primary vascular bundles; 

 /", secondary vascular bundles: /'", leaf -trace 

 bundle within the primary cortex ; 'm, parenchy- 

 tnatous fundamental tissue; s, bund le- sheath ; *, 

 tracheides ; c, cambium ring ; cr, cortex, the outer 

 portion being primary, the inner secondary cortex ; 

 ph, cork cambium ; I, cork ; r, bundles of raphides. 

 (x 30.) 



