446 STUDIES IN FOSSIL BOTANY 



and Taeniopteris, have been found in association with 

 stems of the same type. 



A curious fossil stem, from the Permian of Autun, 

 in France, named by Brongniart Colpoxylon aeduense, 

 and subsequently more fully described by M. Renault, 1 

 agrees in many respects with the simpler forms of 

 Medullosa, but is peculiar in having, for a part of its 

 length, a single vascular cylinder only. The stele has 

 a very irregular outline, and is surrounded by secondary 

 wood and bast of the structure usual in Medulloseae. 

 In the interior there are scattered groups of tracheae, 

 embedded in parenchymatous tissue. 



Towards one end of the specimen, the stele divides, 

 first into two, and then into six or seven parts, so 

 that the stem of Colpoxylon was monostelic in one part, 

 and polystelic in another. The leaf-trace bundles are 

 preserved, and appear to agree essentially with those 

 of Medullosa. Their ultimate branches are collateral, 

 and of the " Myeloxylon " type ; in, some of them, 

 centrifugal as well as centripetal wood has been found. 

 The external surface of the stem bears a general 

 resemblance to that of Medullosa anglica ; the leaves 

 are unknown. There can be no doubt that Colpoxylon 

 belonged to the family Medulloseae ; very possibly it 

 may ultimately prove to have been simply an aberrant 

 representative of the genus Medullosa itself. The local 

 reduction of the vascular system of the stem to a single 

 stele is the point of chief interest. There is some evi- 

 dence that Colpoxylon was the stem of an Alethopteris? 



1 Flore fissile a' Autun et d'Epinac, Part ii. 



2 Grand'Eury, " Sur les organes et le mode de vegetation des Neuro- 

 pteridees et autres Pteridospermes, '' Comptes Rendus, t. cxlvi. p. 1243, 

 1908. 



