OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
695 
the outer bark. I shall be able to demonstrate that this arrangement is not the acci- 
dental result of some disturbing force, but is consequent upon the method in which the 
vascular bundles are given off to the secondary rachis or petioles. As in Rachiopteris 
duplex, these bundles going off to the secondary rachis are given off alternately from 
the outermost vessels of the opposite sides of the central bundle*. A specimen from 
Halifax, in the cabinet of Mr. Carruthers, clearly shows that the new vessels which 
replace those that have now become separated have the closest organic connexion with 
the central bundle, primarily forming a part of it. M. Renault noticed a somewhat 
similar condition in his Zygopteris Lacattii , and rightly suspected that the arrange- 
ment was connected with the formation of the secondary bundles ; but owing to the 
small size of the only fragment which he possessed of this species he was unable to de- 
monstrate this connexion. I happen to have been more fortunate in this respect. 
M. Renault pointed out that all the vessels of the horizontal bar, and most of those 
of the two vertical arcs of Z. Lacattii , were of the reticulated type (“ vaisseaux ponc- 
tues ” or “ vaisseaux areoles). A few only, in each of the lateral arcs, exhibited the 
scalariform type of structure. This result accords with my own observations, only I 
find in my examples rather more barred or quasi-scalariform vessels in the lateral arcs 
than M. Renault appears to have met with. The thin areolae in the walls of the 
vessels are remarkable for their small size and definitely round or oval outlines (Plate 
LVII. fig. 44). The greatest diameter of the vessels is not more than ’0004, and ranges 
between that and -00029. Fragments of this tissue approach nearer to those constituting 
the mineral charcoal so common in the Coal-measures, than any other which I have 
hitherto seen. The innermost cortical layer is very imperfectly represented in my 
examples, this tissue being generally more or less destroyed. Portions of it preserved 
in various specimens, as at fig. 42, g, exhibit a rather delicate form of parenchyma, 
which obviously correspond with that described and figured by M. Renault. That 
observer found the tissue in question penetrated longitudinally by large elongated 
cellules with proper walls, which he believed to have been gum-canals. The middle 
bark ( h ) and the outer one (k) are well preserved in my specimens. In the transverse 
sections the former appears as composed of rather thick-walled cells, which diminish in 
size as we proceed from within outwards. Plate LVII. fig. 43, h , exhibits these cells 
as seen in the vertical sections, where they are arranged in somewhat regular vertical 
lines (orthenchyma). The cells have square ends ; but as we approach the periphery 
they gradually become elongated vertically, and their extremities become pointed and 
overlapping, thus passing into the very narrow elongated prosenchymatous cells ( k ) 
which constitute the outermost portion of the bark. 
* I have recently found a specimen in which these bundles of smaller vessels have been given off simulta- 
neously from opposite sides of the central H-shaped bundle, and are now located in similar positions imme- 
diately within the bark, but in the opposite foci of the elliptical section. In living ferns in which the distichous 
arrangement prevails these secondary rachides are sometimes given off in pairs. The petiole may possibly 
belong to a distinct species with opposite secondary pinnules. — Nov. 9, 1874. 
