466 THE STRUCTURE OF ECONOMIC PLANTS 



divisions wliich proceed until there is a strand several cell layers 

 in width surrounding the duct. (Fig. i.t,S, D.) They later 

 differentiate as the xylem and phloem elements of the mature 

 medullary bundle. 



Medullary bundles are not restricted to the Umbelliferae, but 

 as noted by Wilson (31) are found "in a large number of unrelated 



Fig. 2.38. Medullary bundles: A-C, stages in development of bundle showing central 

 oil duct and amphivasal arrangement of vascular tissues ; D, longisection of portion of oil 

 duct showing formation of provascular strand adjacent to epithelial cells of duct: bu s, 

 bundle sheath ; ept, epithelial cells ; o d, oil duct ; ph, phloem ; p v s, provascular strand ; 

 xy, xylem. 



families, perhaps thirty or more." The term "medullary bundle" 

 has been variously applied by those who have described these 

 structures. They were considered by Weiss (19) and Col (5) to be 

 common rather than cauline in most instances, and were regarded 

 as the extensions of leaf traces instead of an independent system of 

 stem bundles. Westermaier (30), working with species of Begonia 

 which have tubers or rhizomes, suggested that the medullary 

 bundles are supplementary to the usual ring of vascular bundles. 

 In Apium, it appears that the medullary bundles arise early in 

 ontogeny of the stem and may be traced to the ultimate branches 

 of the umbel. It seems probable that they are cauline, supple- 

 menting the common bundles in the translocation of materials 

 during the period of flowering and fruit development. They may 



