SIGILLARIEAE. 259 



English authors, Williamson especially, and brings them into close relations 

 with Lepidodendrae, and through them with Archegoniatae. This divergence 

 of opinion has very recently given occasion to a controversy between Renault 

 and Williamson, which has been already noticed more than once, and which 

 has been conducted by the two opponents in several of the publications 

 cited above. One of the chief arguments advanced by Renault, and drawn 

 from the growth in thickness of the stem, which is supposed not to occur in 

 Archegoniatae, has already been sufficiently examined in the chapter on Lepi- 

 dodendrae. He finds a second and fundamental difference, which ought to 

 prevent any approximation of Sigillariae and Lepidodendrae, in the structure 

 of the leaf-trace-bundles. These are diploxylous in Sigillariae, and such 

 bundles do not occur in Archegoniatae ; Sigillariae therefore cannot belong 

 to Archegoniatae, while Lepidodendrae being Archegoniatae have monoxy- 

 lous bundles. This argument, which is quite analogous in character with 

 the argument from growth in thickness, I must also consider to be incon- 

 clusive, for we have in Isoetes an Archegoniate plant with anomalous 

 position of the initial strand, and there is no apparent reason why there 

 should not have been more of them. Renault himself places Myelopteris 

 among Ferns, notwithstanding that it exhibits the like anomaly. Moreover, 

 as has been already said, we are not yet certain whether the structure of the 

 leaf-trace in Lepidodendrae was collateral or concentric. I incline myself, 

 as I have said, to the view that it was collateral, not only because in 

 Lepidodendron Harcourtii we find a group of bast-fibres in front of one 

 side only, the outer side, of the transverse section of the bundle, but chiefly 

 on account of the character of the traces in Stigmariae, of which we shall 

 have to speak at greater length in the next chapter. But if the bundle in 

 Lepidodendron is collateral, then it is also immediately diploxylous, if 

 Renault is right in his view with regard to the position of its initial group, 

 and every essential difference in this respect between the two groups falls to 

 the ground. If on the other hand it is concentric, then there is really 

 a difference between them, and the only question is as to the importance of 

 this difference ; but on this point there can be no dispute. Lastly, as regards 

 the separate bundle-strands which surround the pith in Sigillaria, their 

 relations to the annular border of tracheides in the central strand of Lepi- 

 dodendron Harcourtii have already been sufficiently considered ; and the 

 like character appears to be found also in one type of Lepidodendron, 

 L. Jutieri, Ren., as is justly and strongly urged by Williamson 1 . It is true 

 that a careful examination of this specimen, which may possibly correspond 

 with Sigillaria Menardi or S. spinulosa before the formation of secondary 

 wood, is much to be desired. When I thus weigh in my own mind all the 

 arguments and counter arguments, and take also into consideration the 



1 Williamson (1\ XI. 



S 2 



