382 Mas leu. — The Structure of Mesoxylou Sutcliffii (Scott). 
The object of the present paper is to give a full account of the structure 
of the stem and part of the leaves of Mesoxylon Sutcliffii , the only portions 
of the plant which are at present known, and to discuss the probable 
affinities of this form. The generic name Mesoxylon has been chosen to 
express the intermediate position of the genus between Poroxylon and 
Cordaites , and its special interest lies in the fact that the different species 
now known appear to almost completely bridge the gap, so far as the 
anatomy of the stem is concerned, between the Poroxyleae and the Cor- 
daiteae, and thus form valuable links in the chain of forms connecting the 
Pteridosperms and Cycadofilices, with which Poroxylon and Mesoxylon have 
much in common, with the more typically Gymnospermous family of the 
Cordaiteae. 
The family Poroxyleae was founded on specimens which were originally 
discovered in the Permo-Carboniferous deposits of Grand’Croix and Autun, 
and which were first described by M. Renault in 1879. 1 In 1886 
appeared the ‘ Recherches sur les Poroxylons * by MM. Bertrand and 
Renault, 2 in which a very detailed account of the anatomy is given, and the 
extraordinarily perfect manner in which the tissues of the vegetative organs 
are preserved is illustrated. In this important memoir a full account is 
given of the structure of the stem, leaf, and root, and to this day it probably 
remains the most minutely detailed account which we possess of the 
anatomy of the vegetative organs of any fossil plant. Several species are 
described, Poroxylon Boysseti (which includes the specimens first discovered, 
and described by M. Renault in 1879), P. Duchartrei , P. Edwardsii , P. 
Stephanense , &c., of which P. Edwardsii and P. Boysseti are the two best 
known French forms. 
The most important characters of the stem of Poroxylon are briefly as 
follows. The comparatively slender stems, less than 2 cm. in diameter, bore 
relatively large simple leaves arranged in a spiral manner and separated by 
rather long internodes, which in the middle region of strongly developed 
branches attained a length of 2 or 3 cm. The leaves themselves were 
simple, broad, and thick, and the lamina was traversed by numerous parallel 
veins. In the axils of many of the leaves the stem bore axillary branches. 
In structure the stem presents a fairly large continuous pith surrounded by 
a distinct ring of collateral leaf-trace bundles, and the development of the 
primary xylem of these bundles is described as being entirely centripetaL 
Each leaf-trace lying on the margin of the pith consists of two bundles which 
remain distinct for some distance below their entry into the interior of the 
stele, finally, however, fusing together, and, after running down through 
1 B. Renault, ( a ) Sur un nouveau genre de tiges silicifiees de l’dpoque houillere. Comptes rendus 
de l’Acaddmie des Sciences, t. lxxxviii, Seance du 6 janvier 1879. (P) Structure comparde de quelques 
tiges de la Flore carbonifere. Paris, 1879, PP* 2 7 0- 8. Nouvelles Archives du Museum, ii, 2 e serie. 
2 Bertrand and Renault, Archives Bot. du Nord de la France, 18S6, pp. 243-389. 
