384 Masleu . — The Structure of Mesoxylon Sutcliffii (Scott)- 
plantes actuelles ne sont pas connus, ce rapprochement ne fixe pas la place 
des Poroxylons dans la classification.’ 
In Renault’s later work he sums up the affinities of Poroxylon and 
states that : ‘ Les Poroxylons sont un type fossile sans representants dans 
la nature actuelle ; ce sont des Phanerogames gymnospermes inferieures, 
plus voisines des Cryptogames vasculaires a structure radiee que nos 
Cycadees, mais superieures aux Sigillaires, aux Sigillariopsis , aux Lygino- 
dendron et aux Heterangimn. Ils n’ont aucun rapport avec les PTicinees .’ 1 
In the light of our present knowledge the affinity of Poroxylon with the 
gymnospermous Cordaiteae on the one hand, and with the pteridospermic 
Lyginodendreae on the other, and so with the Filicineae, appears to be 
indisputable, while that with Sillgiaria and Sigillariopsis is certainly much 
more remote. 
Our very complete knowledge of the vegetative and reproductive organs 
of Cordaites is principally due to the classic researches of MM. Grand’Eury 
and Renault , 2 an epitome of which is given in all the textbooks . 3 
The principal anatomical distinctions between the stem of Cordaites and 
that of Poroxylon are the usually discoid pith of Cordaites , that of Poroxylon 
being continuous ; the denser, more coniferous, wood of Cordaites ; the 
structure of the phloem in Cordaites , different from that of Poroxylon ; 
the division of the double leaf-trace in the pericycle in Cordaites , whereas 
in Poroxylon division is deferred until the leaf has been reached ; and the 
absence in Cordaites of centripetal wood in the stele, the leaf-traces only 
acquiring it on entering the leaf. As Renault states, the stem of Cordaites 
is ‘ absolutely deprived of centripetal wood ’. 4 
As already mentioned, the species of the new genus Mesoxylon appear 
to completely bridge the gap between the Poroxyleae and Cordaiteae. 
They may be said to combine the anatomical habit of a Cordaites with the 
centripetal xylem of a Poroxylon . 
Before passing to the detailed description of Mesoxylon Sutcliffii it will 
be well to repeat here the generic diagnosis of Mesoxylon given in our 
preliminary note. 
Mesoxylon (Scott and Maslen. Annals of Botany, vol. xxiv, 
I 9 IO, p. 237). 
Pith relatively large, discoid . 5 
Wood dense, with narrow, usually uniseriate medullary rays, and relatively 
small tracheides. 
1 Renault, Bassin houiller et permien d’Autun, &c., loc. cit., p. 292. 
2 Grand’Eury, Flore carbonifere du D^partement de la Loire, 1877. Renault, loc. cit., Structure 
compare, &c., 1879 ; Bassin houiller et permien d’Autun, &c., loc. cit., 1896. 
3 e. g. Scott, Studies, 2nd Edition, pp. 518-54. 
4 Renault, Bassin houiller et permien d’Autun, &c., loc. cit., p. 332. 
6 This point is not yet demonstrated in the case of M. platypodium in which the interior of the 
pith is not preserved (see Prel. Note, p. 239). 
