646 THE STRUCTURE OF ECONOMIC PLANTS 



layer, especially in sectors adjacent to the phloem. (Fig. 339.) 

 In addition to the ducts occurring in the pericycle, smaller ones 

 are also found in the medullary phloem strands. They commonly 

 develop around the inner periphery, but may extend through the 

 central portion of the phloem, and similar ducts sometimes occur 

 in the outer phloem. The primary vascular elements of the 

 bundle are annular-spiral or spiral, the latter type frequently 

 forming walls w^ith double spiral thickenings. Slender, elongated 

 parenchymatous cells separate the vessels from each other. The 

 secondary vessels are large and angular with transversely elongated 

 pits which sometimes give the vessel segments the appearance 

 of being scalariform or reticulate. 



Medullary Bundles. — Medullary phloem strands appear in 

 the pith centrad to the primary xylem so that the bundles may 

 appear to be bicollateral. However, the late development of the 

 medullary phloem strands, in addition to the fact that their 

 arrangement and the number of strands adjacent to a bundle may 

 vary, indicates that these units do not constitute an integral part 

 of the vascular bundle. 



With respect to this situation Solereder (15) has pointed out 

 that in the Cichorieae there may be medullary vascular bundles 

 formed by the addition of xylem to the phloem. This was 

 observed by Petersen (lo), who noted intraxylary phloem in 

 Lactuca and described the occurrence of some xylem in connection 

 with this tissue. Weiss (30) also investigated this matter and 

 demonstrated that the chief bundles of the vascular system which 

 have inner phloem associated with them are not true bicollateral 

 bundles. Specifically referring to L. sativa, he stated that the 

 medullary phloem is directly continuous with the leaf trace in 

 some instances, and that the small lateral phloem strands adjacent 

 to the protoxylem groups are direct continuations of strands 

 arising on the adaxial face of the leaf bundle. Other phloem 

 strands were found to pass into the pith as branches from the outer 

 phloem of the stem bundle, so that they could be regarded as being 

 only indirectly connected with leaf traces. Kruch (16) also 

 investigated this group extensively, and concluded that the medul- 

 lary strands might be direct continuations from the phloem asso- 

 ciated with leaf or branch traces. In a few cases, including 

 Lactuca, there are both foliar and cauline strands. 



Worsdell (31), having studied medullary phloem in the Com- 



