174 KI.KMKN'TH OF STRUCTURAL BOTANY. 



the cambium divide, and the. 7io\v cells thus continually 

 being formed become tnodiiied on tlie one hand into i 

 \vliicli increase the thickness of the xylem, and, on tlio 

 other, into tissues which are added to the phloem. Liter 

 on cambium cells are formed inrthe ground-tissue b<- 

 the bundles, thus linking together the cambium-lay 

 the various bundles, and forming a continuous ring. The 

 links are then known as interfascicular cambium, that of 

 the bundles themselves being the fascicular. Bundles of 

 this kind, characterized by the cambium-layer, and so 

 capable of continuous enlargement, are called open bundles. 



298. In monocotyledons, on the other hand, there is no 

 cambium-layer, and consequently the bundle when once 

 formed is incapable of further in- 

 crease, and so is said to be cl-- 

 Fig. 229 is a representation of the 

 cross section of an endogenous sli-ni 

 in which many of these closed bundles 

 are visible. Of course in such stems 

 no bark is formed. 



299. It has been explained that 

 in the exogenous stem the xylem occupies one side of the 

 fibro-vascular bundle, while the phloem occupies the other. 

 In the closed bundles of Ferns and Club Mo>-es, as well 

 as of some monocotyledons, however, a different arrange- 

 ment prevails, the xylem occupying the central part of 

 the bundle, and the phloem forming a circle around it. 

 The former arrangement is described as collateral, while 

 the latter is concentric. In many of the monocotyledons, 

 as well as in the exogens, the bundles ;uv collateral. 



Fig. 229. Cross-section of monocotyledonous su-m, showing rlosul luiiidks. 



