ORGANIZATION OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 731 
“ Rachiopteris aspera.” We therefore know that these leaflets really belonged to our 
plant. Specimens in which such details of the leaf-structure are well preserved, are 
necessarily rare. 
A vertical section such as those represented, shows first of all, that the structure of 
the lamina was distinctly bifacial or dorsiventral. Towards the upper convex surface 
of the leaflet, the mesophyll consists of closely packed cells, elongated at right angles 
to the surface. Towards the lower concave surface, the cells, though still somewhat 
elongated, become more irregular, are sometimes branched, and leave considerable 
intercellular spaces between them. 
Where the section passes through a vascular bundle, we see that the latter is 
enclosed within a very definite parenchymatous sheath, consisting of large cells 
elongated parallel to the bundle. We have endeavoured to find out whether the fine 
bundles of the lamina are concentric or collateral, a question of considerable difficulty. 
In one or two cases we have seen that the spiral-tracheides, of which the xylem is 
exclusively composed, are in direct contact with the bundle sheath on the side towards 
the upper surface of the leaf, while on the lower side some intermediate thin-walled 
elements could be recognized (see fig. 16; C.N. 1856 shows this point well). Most 
probably the bundles of the lamina were collateral, as in recent Ferns, though 
throughout the rachis they were certainly concentric. The trachez became dilated in 
some cases towards the ends of the veins. This dilatation may have been connected 
with the presence of a water-gland.* : 
We were unable to determine the position of the protoxylem in the vascular 
bundles of the lamina, 
The leaf possessed a distinct hypodermal layer beneath the epidermis of the upper 
surface (see photograph 7 and fig. 16). The epidermis itself is not so well-preserved 
as the mesophyll, but in one or two places the stomata can be recognized. The clearest 
is seen in sectional view in fig. 164. Here there seems to be no doubt, that we have 
the two guard cells (g.c.), lying at the base of a depression formed by the prominent 
subsidiary cells. We have so far found no trace of stomata on the upper surface of 
the leaf. 
The lamina of the leaf occasionally has outgrowths on its lower surface, similar to 
those on the petiole and stem (see photograph 7 and fig. 16). It is a curious fact, 
that in the best sections of the leaf, almost every cell contains a round mass of 
carbonaceous matter, very suggestive of a nucleus. We do not desire to lay any 
stress on this appearance, but its constancy in the best preserved preparations is 
certainly remarkable. (See photograph 7 and fig. 16.) 
The following conclusions result from our observations :— 
1. The lamina of the leaf in Lyginodendron had a distinctly bi-facial structure with 
well differentiated palisade and spongy parenchyma. 
* O.N, 1856. See Poirautr, ‘Recherches sur l’anatomie des Cryptogames vasculaires,” ‘ Ann. des 
Sci. Nat.,’ Bot., Sér. 7, t. 18, 1894. 
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