682 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
tuberances. These are generally extensions of the middle parenchyma rather than of 
the prosenchymatous cells, and obviously correspond with similar projections seen on 
the petioles of many recent ferns, as in Alsophila excelsa and other arborescent forms. 
Brongniart has also figured several fossil species of Pecopteris in which the rachis is 
covered with similar protuberances. I presume they are to be regarded as undeveloped 
hairs, or as the remnants of abraded ones. 
Having thus ascertained the structure of the matured petiole, we may now examine 
the modifications which that structure undergoes as we proceed from the base towards 
the tip of the rachis. As we do so we find that the horizontal bar of the vascular 
bundle, seen in transverse sections like fig. 1, gradually diminishes in size and alters 
its form until we obtain the condition represented in Plate LII. fig. 6, a , where we have 
two bundles. These are widely divergent superiorly, but almost meet dorsally. In 
Plate LII. fig. 7, these two bundles have reunited to form the gutter-shaped bundle of 
Trecul. In Plate LII. fig. 8, this bundle is but slightly cordate, whilst in figs. 9 & 10 
(which latter represents the extremity of the rachis) we have only a very small cylin- 
drical bundle remaining. In all these sections we see in the bark the same features 
noticed as existing in fig. 1, but in a more prominent form. In the upper part of 
fig. 6 the cells of the middle parenchyma ( h ) are somewhat drawn out radially, whilst 
the prosenchymatous fibres (k) are very clearly shown, their cavities being filled with 
coally matter. In the smaller sections 7, 8, 9 & 10, the abortive hairs ( k ") become con- 
spicuously large in proportion to the size of the petiole. This feature reappears in all 
the smaller sections*. All these sections are drawn to one scale. 
Some longitudinal sections which I have made of these smaller parts of the petiole 
reveal some important features. Plate LII. fig. 11 represents one of these, and fig. 12 
a second one still more enlarged. The vascular bundle reduced in size appears at a. 
In fig. 12, b , we find traces of the innermost cortical parenchyma, which here appears to 
consist of a few layers of narrow, vertically elongated, square-ended cells, suggestive of 
a rapid vertical growth. The middle parenchyma (A) resembles that seen in sections 
made near the thicker base of the petiole, but in these terminal portions a remarkable 
difference presents itself : we find bands of small, compactly grouped cells (i) extend- 
ing from the inner to the outermost bark ; and at i', where the section has passed 
somewhat tangentially between the inner and outermost bark, we see that these cells 
are frequently arranged in quoit-shaped disks, exactly resembling, save in their greater 
irregularity of position, the similar disks in the young petioles of Heterangium Grievii , 
described in my fourth memoir (Phil. Trans. 1873, PI. xxxi. figs. 45 & 47). I think 
there can be no doubt whatever that these are the structures which, breaking up at a 
later period of growth into detached masses, and having their special cells subse- 
quently absorbed, produce the intercellular lacunae (i) seen in Plate LI. figs. 2 & 4 ; 
at least I have failed to find any other explanation of the existence of these two singular 
arrangements in the young and tlie more matured parts of the petiole. 
* Bbongniaet applies to these organs the name of “ ecailles setacees ” (Yeg. Fossiles, p. 200). 
