356 REMAINS OF STEMS OF DOUBTFUL AFFINITY, ETC. 



towards the point.' Other modes of explanation, for example the supposition 

 of an alternation of leaves, seem to me to be preferable. The transverse 

 section of the trace-bundle, according to the figure and the description 1 , 

 shows a structure essentially similar to that of Cordaites ; it is surrounded by 

 a stout parenchymatous sheath, and exhibits the initial group characterised 

 by spiral tracheides between the two wood-strands, the upper one of 

 which forms an irregular group and is surrounded on one side by the lower 

 strand which is in the form of an arch. A hypodermal fibrous layer with 

 blunt projecting ribs is developed beneath the epidermis on both sides 

 of the leaf. I have been obliged to depend entirely on Renault's description 

 for this interesting genus, which I have never examined myself; a fuller 

 account of it from better preserved specimens would be very desirable. 



The remains which Renault 2 has collected together provisionally under 

 the name Poroxylon, and which differ from one another in not unim- 

 portant points, are also known to me only from the literature. There is a 

 prospect of a full account of them by Bertrand and Renault, the main results 

 of which have already been published in a short preliminary communication 

 by Bertrand 3 . According to Renault's figures we must here distinguish 

 two separate types, one of which is represented by Poroxylon Boysseti and 

 P. Edwardsii, the other by P. Duchartrei. While the first two forms, 

 judging by the figure and the description, appear to have much resemblance 

 to Sigillariopsis, and show pith and rind and between them a ring of bundles 

 surrounded by secondary wood, the figure of P. Duchartrei 4 has a central 

 wood-strand of circular transverse section, in which groups of broad pitted 

 tracheal elements are imbedded in a delicate parenchyma. This central 

 strand is surrounded by a ring of secondary wood divided by very broad 

 medullary rays into a great number of wedges, which in their turn contain 

 narrower rays of parenchyma. The tracheides are pitted. Few scalariform 

 or spiral cells were found on the borders of the central strand. Nothing 

 remains of the rind except a few fragments of secondary bast-wedges an- 

 swering to the segments of the ring of wood. 



It is possible that Bertrand's and Renault's 5 statements refer only 

 to the type represented by the first-named species. At least I am unable 

 to reconcile the following words with the structure of Poroxylon Duchartrei : 

 ' The centripetal ligneous masses (the primary bundles) do not converge to- 

 wards the centre of the stem, even in slender stems.' This quite suits the first 

 two species which resemble Sigillariopsis. In these there is a central pith, 

 and in it in P. Edwardsii there are dark points, which Renault takes for 

 gum-passages. Similar passages are found in P. Boysseti in the paren- 

 chymatous rind. On the inner border of each segment of the ring of wood 



1 Renault (1), 1. 13, ff. 1-3. a Renault (1), p. 272, and (2), vol. i, p. 119, t. 16. 8 Bertrand (3\ 

 * Renault (1), t. 14, f. 4. Bertrand et Renault (3). 



