174 ELEMENTS OP STRUCTURAL BOTANY. 



the cambium divide, and the new cells thus continually 

 being formed become modified on the one hand into tissues 

 which increase the thickness of the xylem, and, on the 

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

 on cambium cells are formed in the ground-tissue between 

 the bundles, thus linking together the cambium-layers of 

 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 closed. 

 Fig. 229 is a representation of the 

 cross-section of an endogenous stem 

 in which many of these closed bundles 

 are visible. Of course in such stems 

 no bark is formed. 



Figl 229> 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-Mosses, 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 are collateral. 



Fig. 229. Cross-section of monocotyledonous stem, showing closed bundles. 



