870 SUMMARY OF CURRENT RESEARCHES RELATING TO 



bundles of the peripheral ring pass in the next node above into the 

 leaves, and the medullary bundles are arranged between bundles of 

 the peripheral ring (Begonia'), or in most cases, form the direct con- 

 tinuation of leaf-traces, which after their entrance into the stem, run 

 through one or more internodes in the perijiheral circle, before they 

 bend into the pith. In the exogenous formation of bundles, the leaf- 

 traces pass into the pith immediately after their entrance into the 

 stem, and bend towards the periphery only in lower nodes. Neither 

 the exogenous nor endogenous mode of formation of bundles is 

 properly included under the term cauline. 



The bundles which occur in some Begonias are not cauline ; those 

 so-called in this genus pass in the node into the peripheral ring, and 

 bend into the leaf in a still higher node. The medullary bundles in 

 some species of Aralia, as A. edulis and racemosa, run through one inter- 

 node in the peripheral ring, after their entrance into the stem, before 

 they bend into the pith in a lower node, in consequence of the entire 

 bundle twisting through an angle of 180^. Silaus pratensis exhibits 

 the same phenomena, only that there is no twisting of the bundle. In 

 all other Umbelliferfe with a medullary system of bundles, it may be 

 assumed that they are simply continuations of the leaf-traces. 



The medullary bundle-system of Tecoma radicans is the immediate 

 continuation of the median bundles of the leaf-trace, which after their 

 entrance from the leaf, run in the peripheral ring through two inter- 

 nodes, before they pass into the pith in the third node, accompanied 

 by twisting through 180^. Species of Acanthus, as A. longifolius, 

 exhibit the same peculiarity. 



The medullary vascular (often only phloem-) bundles in some 

 species of Campanula are not cauline ; their course is the same as in 

 Tecoma and Acanthus. In C. pyramidalis there is often a closed ring 

 of vascular bundles composed of several medullary continuations of 

 leaf-traces, which spring partly from the median leaf-trace of the next 

 leaf above, but mostly from those of leaves of still higher insertion. 

 A similar course is taken by the medullary phloem-bundle of 

 Cichoriacefe, as in Scorzonera, Sonchus, Lactuca, &c. They are 

 occasionally also accompanied by xylem, and, in the lowest internodes, 

 become concentric vascular bundles. 



The medullary bundle in ConvolvulaceaB, Apocynaceae, Solaneae, 

 Myrtacete, Gentianacese, &c., is never cauline, it always passes, with 

 the vascular bundles within which it lies, into the leaves, and 

 unites, as it expands in the lamina, with the peripheral bundle of the 

 vascular bundles. The phloem-bundles of these families differ 

 from those of Cichoriaceae by their course and by the absence of 

 xylem. 



The medullary vascular bundles of Melastoraacese are also not 

 cauline, but are the direct continuation of the phloem-bundles which 

 run at the margin of the pith in the higher internode, and are often 

 accompanied by annular and spiral vessels when they pass into the 

 node in the more central part of the pith. The arrangement of their 

 xylem and phloem is not concentric. 



In thick fleshy roots the medullary vascular bundles of the stem 



