HICK AND CASH : FLORA OF LOWER COAL MEASURES. 403 
Between these filaments are spaces destitute of tissue of any kind, 
which are obviously the sections of large air cavities, which run 
longitudinally through the stem. The longitudinal section is hardly 
so demonstrative as we could have wished, but it leaves no doubt 
on our minds that the radiating filaments are merely the trans- 
verse appearances of longitudinal plates of parenchymatous tissue, 
that form the lateral boundaries of the air cavities. As we have 
mentioned above, these plates do not anastomose with one another, 
but run free from the central axis in radial directions. 
The outer part of the cortex is not well represented in any of 
our preparations, the stem having been compressed and probably 
undergone some surface disintegration previous to fossilization. 
Moreover it is so closely intermixed with other fragments of veget- 
able tissue, that its limits are not strictly definable. Nevertheless, 
enough of the peripheral cortex still remains to enable us to state 
that it was partly, and most probably, wholly cellular. 
3. Probable Affinities. — In venturing to speculate on the prob- 
able affinities of the plant, whose stem has the structure and 
organisation j ust described, we are not unmindful of the difficulty, 
not to say danger, of so doing without a knowledge of the structure 
and arrangement of its other organs. Undoubtedly in the absence 
of these it is impossible to determine with certainty its systematic 
position, or even to speak with full confidence of the class to which 
it must be assigned. But this we opine is no reason why we should 
refrain from at least pointing out the resemblances which we find 
between the organisation of this stem, and that of certain recent 
forms with which all botanists are familiar. We allude to such 
well known forms of acquatic plants, as Myriophyllum, Hippuris, 
Potamogeton, and a few others. In most of these the stem exhibits 
an axial string of fibro-vascular bundles, surrounded by a cortex 
of comparatively large dimensions, which is traversed longitudinally 
by a number of large cavities. These cavities are divided one from 
another by parenchymatous lamellae, one cell thick, which in some 
cases run direct from the axial string to the outer layers of the 
