ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 729 
bend at the middle of the xylem and one near each end; in larger bundles they were 
more numerous. The smallest elements were not situated at the periphery of the 
xylem, but were imbedded in its mass, nearer the lower than the upper surface. That 
the elements in question are really the spiral tracheides or protoxylem is proved by 
the longitudinal sections (see Plate 23, fig. 13, px., cf: also C.N. 1982). 
We thus see, that the position of the initial strand is essentially the same as in 
the bundles of the stem. We desire to call special attention to the fact that there 
is not the slightest reason for regarding the centrifugal or lower portion of the xylem 
in the leaf* as secondary, which is the view of MM. Bertranb and Renavtt in the 
ease of Poroxylon. In Lyginodendron this tissue shows no sign of radial seriation. 
and consists mainly of spiral or reticulate elements, while pitted tracheides chiefly 
occur in the centripetal portion of the xylem. In Poroxylon, true secondary wood 
was no doubt formed in the leaf, but we do not feel certain from the figures that 
there may not have been some primary centrifugal wood here also.t 
The wood of the petiole resembles the primary wood of the stem except that it has 
a larger proportion of spiral and reticulated as compared with pitted elements. A 
certain amount of thin-walled parenchyma was present among the tracheides. It is 
probable that a layer of parenchyma also separated the tracheides from the phloém. 
The phloém, as seen in longitudinal section, consists of very elongated elements, 
probably the sieve-tubes, and of shorter parenchymatous cells (see fig. 13, ph.). 
The great point to be emphasized as regards the bundles in the petioles of 
Lyginodendron is, that they are the typical concentric bundles of a Fern, whereas 
those in the stem have a distinctly Cycadean character. 
We have preferred to use the old term concentric bundle rather than stele in this 
case, because the vascular tissue of the petiole forms the direct continuation of the 
collateral strands of the stem, which no one could regard as anything else than vascular 
bundles of the usual type. 
The inner cortex of the petiole presents one or two points of interest. It is 
traversed longitudinally by great numbers of those elongated tubes with carbonaceous 
contents which we regard as being of the nature of secretory sacs. The individual 
sacs must have been of great length in the petiole, for their terminal walls are rarely 
met with. They sometimes anastomose with one another, but we cannot say 
whether an actual fusion ever took place. The cortical parenchyma is crossed in a 
horizontal direction by transverse bands of short sclerotic cells. These often form, 
together with the longitudinal secretory organs, a most conspicuous network.{ It 
* In every petiole of Lyginodendron the upper and lower side can easily be determined in any trans- 
verse section. The upper side is flat or concave, the lower side is convex. The form of the bundle or 
bundle-pair follows the same rule (see Memoir VI, figs. 1, 6, 7, Memoir XVII, Plate 13, fig. 7, also 
our photographs 3 and 5 and fig. 10). 
+ See Berrranp and Renavtr, ‘ Sur les Porowylons,’ figs. 219, 220. 
+ Something is shown of this on a small scale in fig. 11. This and many other anatomical details 
could not be adequately figured without increasing the number of illustrations beyond all bounds, 
MDCCCXCV.—B, 5 B. 
