434 Boodle .• — The Mode of Origin and the Vascular Supply of 
whicli spiral thickening was present having been observed. The end-walls 
of the segments of the vessels have each a simple perforation, which is 
often small, and may not be recognizable in an optical longitudinal section 
of a vessel. The general appearance of some vessels is shown in Fig. 6, 
which represents a small portion of a leaf-trace. The phloem-strand (ph.), 
indicated in the figure, appears discontinuous owing to the irregularity of 
its course. 
Transverse sections of the leaf-trace may present very different 
appearances at different points in its course through the cortex. In one 
case, for instance, the petiole has a typical arched bundle, which, on entering 
the tuber as the leaf-trace, shows rearrangement of its tissues, the xylem- 
elements first changing their course so as to be cut longitudinally in 
a tangential section of the tuber, thus giving the appearance of a plate of 
xylem. By a further change of direction there is an almost immediate 
Fig. 5. Fig. 6. 
Figs. 5, 6. 5. Longitudinal section of tuber, partly diagrammatic, showing vascular 
supply of adventitious leaf; x. and ph., xylem and phloem of stele ; x . 1 and ph. 1 , xylem and phloem 
of leaf-trace ; p., pith ; /., leaf, x about 22. 6. Portion of trace of adventitious leaf cut 
longitudinally; ph., phloem, x 260. 
return to the arched form of bundle, the diameter of the arch being greater 
than before, and the xylem becoming somewhat interrupted. The trace 
then, by a slight change, assumes the form of a more or less broken ring of 
xylem and phloem (ectophloic), and retains this form for some distance, 
but with irregularities in the course of its tissues. After becoming once 
more arched, the trace divides into two or three strands preparatory to 
fusion with the central cylinder. The phloem-strands are sometimes rather 
widely separated from the xylem. 
In one or two cases the petiolar bundle was not arched, but had a small 
solid cylinder of xylem surrounded by phloem, and this type of structure 
also was met with in portions of some leaf-traces. Small strands connecting 
adjoining traces sometimes occur, and, when several leaves are inserted close 
