886 



A TEXTBOOK OF THEORETICAL BOTANY 



cambium, so that they retain their initial breadth permanently, but in timber 

 trees the original broad rays are soon bridged and closed by the interfascicular 

 xylem. Certain interfascicular cambial cells, however, produce only 

 parenchyma cells, both inwards and outwards, so that at these places narrow 

 rays are built up, which may be uniseriate, or at most two to three cells broad. 

 These narrow rays continue outwards through the whole zone of vascular 

 tissue, no matter how broad it may become, from the pith to the pericycle, 

 interrupted only by the cambium itself. 



Medullary rays which originate after secondary thickening has begun 



■4~i Cortex 



I External phloem 



.M,.^ Cambium 



Metaxylem 



Protoxylem 



Internal phloem 



«>!*<,% 



1 



Fig. 872. — Cuciirhita pepo. Transverse section 

 of a bicollateral primary bundle of the stem. 

 The inner side of the bundle is bounded by 

 the space left after the disappearance of the 

 pith. 



are called secondary rays. Their point of origin is an initial parenchy- 

 matous cell, cut off, usually terminally, from one of the fusiform cambial 

 cells. 



Similar narrow rays are formed in the original bundles as soon as radial 

 growth from a cambium is established, and they may be similarly permanent. 



The details of secondary growth we will leave until later, while we con- 

 sider some characters of the individual bundles. 



Two types besides the simple collateral bundle deserve mention. The 

 first is the bicollateral, in which there is a second phloem region, usually 

 smaller than the first, inside the xylem zone (Fig. 872). This internal or 

 medullary phloem may be confined to the primary bundles, or it may 

 develop in the interfascicular region as well, so that a more or less continuous 

 zone of internal phloem resuhs (Figs. 873 and 874). It is a character found 





