680 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
sheath, or of any fibrous elements surrounding this vascular bundle. As is so frequently 
the case amongst the half-dried petioles of recent ferns, the surrounding parenchymatous 
cells have slightly shrunk away from the bundle in several parts ; but in others they 
extend up to it. The vertical and horizontal portions are all slightly curved the 
concavities of the former being directed outwards, and that of the latter to the lower 
or dorsal surface of the petiole. Those who are familiar with the aspects of recent 
petioles of ferns will have no difficulty in identifying the superior and dorsal surfaces 
in this example. Much of the inner and middle portions of the cortical parenchyma 
(g and h) has been destroyed in the specimen ; but what remains suffices to show that it 
was of the ordinary type, and composed of cells whose diameters ranged about *0025 
to ’003, and, so far as this individual section indicates, of nearly uniform character 
throughout its entire extent. Other sections will show that such was not always the case. 
The exterior of the petiole consists of a denser layer {Jc), having a thickness of from ‘012 
to -016. This is composed of alternating wedges of sclerenchyma {Jc!) and parenchyma 
( Ji '), the broader parts of the former of which and the narrower ones of the latter reach 
the peripheral margin of the section ; but the forms as well as the sizes of both are 
irregular and unequal. The sclerenchyma (Jc') consists of narrow prosenchymatous cells, 
of from •0012 to -0016 in diameter. Their internal cavities are generally filled with 
carbonaceous matter, making them black and opaque. The intervening angles between 
the wedges {Jc’) are occupied by an outward extension of the middle parenchyma (h 1 ), 
which here reaches the surface of the petiole. These cells are smaller and more uniform 
in size than in the interior of the section. The peripheral outline projects at numerous 
points {Jc ", Jc") into little swellings. These we shall afterwards find are the little warty 
projections from the surface of the petiole, apparently abortive hairs, to which I have 
already alluded. Transverse sections, like that just described, sometimes intersect dark- 
coloured intercellular lacunae ; but these are much better seen in the vertical sections. 
Plate LI. fig. 2 is a vertical section made a little obliquely through a section like 
the last, a represents the central vascular bundle, g the innermost parenchyma of 
the cortex, Ji its median portion, and Jc its prosenchymatous investment. The vessels 
composing the central bundle in this and all similar specimens are either reticulate (Plate 
LI. fig. 3 A), barred (fig. 3 B), or spiral (fig. 3 C). The greater portion of them belong 
to the first of these forms. This is especially the case with those occupying the central 
parts of all longitudinal sections. Towards the outer margins of the bundle in such 
sections we find some that are barred and a small number that are unmistakably 
spiral vessels. The larger ones are invariably reticulated, and in passing along the series 
we find connecting links between them and the smaller barred ones. On turning to 
transverse sections of similar bundles, we find that the central line of each bundle is 
occupied by vessels of larger size than its more peripheral ones. These facts accord 
substantially with the observations of M. Trecul upon recent Ferns, and which accord 
with my own. The spiral and annular vessels are both of earlier growth, and occupy 
a more peripheral position, as well as being of smaller size than the larger scalariform 
