383 



EOUISETALES 



the number of the tracheides in these varies in different species 

 (Fig. 211 B, D, F). It will be shown that these are not directly con- 

 tinuous with the xylem of the leaf-trace. The phloem lies between them, 

 and consists of sieve-tubes and parenchyma. 



If the vascular tissues be followed omvards into the nodes, the structure 

 there displayed will give ground for a proper understanding of the inter- 

 nodal strands. Hitherto it has been customary to treat these as integral 

 "vascular bundles" of the collateral type, comparable with the leaf-trace 

 bundles of Phanerogams : they have been assumed to enter the axis from 

 the leaves as integral bundles, and to pursue their course down one 



internode, maintaining their identity as 

 integral bundles to its base: there 

 each was held to bifurcate, and the 

 shanks to affix themselves right and 

 left on the nearest lateral bundles 

 which pass in at the lower node. This 

 was the scheme contemplated by De 

 Bary ; 1 but it is a scheme characteristi- 

 cally Phanerogamic, and it has always 

 presented difficulties of comparison with 

 other Pteridophyte-types. An advance 

 to a more intelligible view, based upon 

 more exact analysis of the nodal 

 structure, has been the result of the 

 investigations of Gwynne-Vaughan.- to 

 whom I owe the use of hitherto un- 

 published drawings, as well as the 

 description which follows. He found 

 that in E. Tclmateja, of the three 

 strands of xylem present in each bundle 

 of the internode the carinal strand alone 



passes out at the node as a leaf-trace. The two lateral strands join on 

 to the xylem of the nodal ring, where the xylem is much more amply 

 developed than in the internode, and even shows some slight degree of 

 secondary increase. 3 In certain species (E. liieinale, and better still in 

 E. giga/ifeum} the lateral strands of the internodal bundles may be traced 

 as externally projecting ridges over the nodal xylem into the internode 

 above.* In passing through the node they diverge from one another, so 

 that in the internode above they are found on the adjacent sides of two 

 different bundles. At the node above they approach each other, and in 

 the next internode they both occur in the same bundle once again. The 



1 Comp.-Anat., pp. 279 and 327. 



2 Gwynne-Vaughan, Report Brit. Ass., < lla^nw, 1901, p. 850; also Ann. of Bot.. 

 1901, p. 774. 



"Cormack, Annals of Botany, vii., p. 63. 



FIG. 212. 



Diagram constructed by Mr. Gwynne-Vaughan 

 to represent a tangential view of the vascular 

 system of EquisetuiH. The dotted lines indicate 

 the course of the true leaf-trace strands: the 

 continuous lines indicate the cauline strands. 



