SECT. I 



MORPHOLOGY 



139 



fundamental tissue, it is a secondary meristem ; it does not, as in 

 Dicotyledons and G-ymnosperms, produce continuously wood and bast 

 in opposite directions, but, instead, closed vascular bundle strands and 

 fundamental tissue (Fig. 154). 



?* 



The cells arising from the division of 

 entirely towards the centre of the stem, 

 by means of variously disposed 

 longitudinal walls, and produce 

 new vascular bundles (/"), or, by 

 forming tangential and transverse £ 

 walls only, they give rise to the '- 



radially arranged cells of the funda- 

 mental tissue, which fills the space 

 between the vascular bundles. 

 These secondarily developed bun- 

 dles, like the primary bundles, are 

 closed, that is, they do not possess 

 a cambium, but have nevertheless 

 a somewhat different structure. 

 Their xylem portions consist solely 

 of tracheids provided with bordered y ' 

 pits, and completely enclose the 

 thin-walled and sparingly deve- 

 loped phloem. Towards the peri- 

 phery of the stem the cambium 

 ring produces only a small amount 

 of parenchymatous tissue, the cells 

 of which sometimes contain bun- 

 dles of raphides (r). A stem of 

 a Dracaena having this form of 

 secondary growth may attain a, 

 considerable thickness. 



Periderm. — It is very 

 seldom that the epidermis, 

 by the division of its own 

 cells, is in a condition to keep 

 pace for any length of time 

 with the increasing dimen- 

 sions of the stem. This, 

 however, is the case with the 

 Mistletoe (Viscum album), the 

 number of whose epidermal 

 cells is continually augmented 

 by the formation of new lateral 

 walls, while the outer walls 



the cambium cells (c) are given off 

 The new cells thus derived either 





almost 

 divide 



)' 



SS^EBtt- ''■ 







-f 



Fig. 154. — Transverse section of the stem of Cordylinc 

 (Dracaena) rubra, f, Primary vascular bundles ; 

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

 bundle within the primary cortex ; m, parenchy- 

 matous fundamental tissue ; s, bundle-sheath ; t, 

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

 portion being primary, the inner secondary cortex ; 

 ph, cork cambium ; 7, cork ; r, bundles of raphides. 

 (X 30.) 



are at the same time strengthened by inward thickenings to supply 

 the place of the older, ruptured, thickening layers. The stems 



