OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
201 
section of the outer bark passing through the prosencliymatous layer and immediately 
underlying the epidermal one ; but owing to the cylindrical form of the specimen, on 
the left hand of the sketch the section has passed outwards through the latter layer 
(fig. 5, l ) and the attached bases of the leaves. The prosencliymatous character of this 
bark-tissue is well shown in this section. Openings, indicating the points through which 
the foliar bundles of vessels have passed, are seen to be partly occupied by delicate cells. 
The section of each opening is oblong in a vertical direction. Fig. 6 is a tangential 
section made parallel to the last, but through the outermost epidermal layer ( l ). The 
bases of the leaves are here indicated by large lozenge-shaped spaces (fig. 6, l), arranged, 
like the corresponding openings in fig. 5, in quincuncial order. These two sections 
illustrate, with great clearness, the tissue to which two common appearances seen 
amongst fossil Lepidodendra belong. Fig. 5 represents the Knorria - like forms which 
are commonly, but erroneously, spoken of as decorticated ; whereas they belong chiefly 
to the outermost surface of the middle cortical layer, the bast-layer and epidermis alone 
having disappeared : such a surface, in the plant under consideration, is represented in 
fig. 7. In some larger stems of this, or an almost identical species, belonging to J. B. 
Dawkins, Esq., the lenticular projections are rather shorter and broader than those in 
this figure. Fig. 5 corresponds to the ordinary Lepidodendra. 
The sections of the persistent bases of the leaves ( l ) vary considerably in form, as is 
shown by figs. 1,2, 5, & 6. So far as the transverse section is concerned, fig. 1, l" , 
appears to indicate the characteristic form, since its resemblance to a depressed acumi- 
nate leaf reappears with more or less of distinctness in most of the other sections made 
in the same plane. These petioles consist of coarse thick-walled parenchyma, the cells 
in some portions not unfrequently appearing elongated in the direction of growth. The 
cells of the exterior surfaces are small and dense. 
I have mentioned that in the medullary axis of the specimen described there is a very 
distinct appearance of two kinds of cells, but I am far from certain respecting the true 
signification of this difference. In other sections of the same species of Lepidodendron 
I find similar appearances, but with more semblance of a transition from the barred and 
thick-walled to the thin-walled cells ; whilst in one specimen, the centre of which, how- 
ever, is considerably disorganized, every cell, large and small, appears as it it had been 
equally barred. I have long since learnt that amongst these coal-plants the absence of 
a barred or reticulated structure from a cell or vessel is no proof that such secondary 
elements never existed. We frequently find that, during mineralization, the carbonaceous 
matter, representing the original deposits of lignine, has been diffused in a uniform, gra- 
nular layer over the walls of the tissues. Hence it is barely possible that the variations 
in the medullary cells of the plant described are due to such mineralization. 
It appears that, in this plant, we start at the centre with a highly vascular axis inter- 
mingled with cellular tissue, and that the vessels, though diffused over the entire medulla, 
exhibit a slight tendency towards a peripheral polarity, being less intermingled with 
cells at the exterior of the medulla than at its centre. Around this we have a thin layer 
