LEPIDODENDREAE. 22$ 



though Binney 1 has figured a transverse section of a stem of the vasculare- 

 type, in which the central strand, seven millimetres in thickness, is quite 

 insignificant as compared with the secondary wood, which is sixty-two 

 millimetres thick. The disposition also of the tissue of the primary wood 

 in this specimen is somewhat abnormal. 



These stems with secondary wood have hitherto been unhesitatingly 

 regarded as more advanced states of development of Lepidodendron. It 

 is necessary however to show some reasons for this view, since we know 

 that no such secondary formations occur in recent Archegoniatae, or only in 

 a quite rudimentary form in Isoetes. Secondary wood is said by recent 

 authors to be found in older stems of Botrychium, but the point requires to 

 be cleared up by further investigation. The view which Renault has 

 developed in conjunction with Brongniart, that the presence of secondary 

 xylem must exclude any form from the class of Archegoniatae, is at once 

 refuted by the case of Isoetes. For if it can be shown that the character is 

 present in the class in one instance only and in ever so rudimentary 

 a form, it is difficult to see why it should not have occurred fully developed 

 in other extinct representatives of the class. And when we see this addi- 

 tional character present in so many specimens, which perfectly agree with 

 Lepidodendron in all points of structure and surface-features, it seems to me 

 that we shall be doing violence to nature if we are determined to keep them 

 separate from that genus, and place them in another group of the vege- 

 table kingdom for the sake of some view once adopted which has become 

 a favourite with us. Now this is Renault's position, when he refers the 

 remains in question to Sigillarieae and with them to Gymnosperms, and 

 Williamson 2 is quite right in maintaining that his classification rests entirely 

 on a petitio principii ; nor can I help acknowledging that I am of his opinion 

 after repeated careful study of the numerous specimens in his own possession 

 and in that of Carruthers, Cash, and myself. Renault has naturally en- 

 deavoured to support the view which he received from Brongniart by as 

 many further arguments as he could command, and their unsatisfactory 

 character has been also exposed at length by Williamson and Hartog 3 . 

 Renault's prime contention is, that the surface of the specimens examined 

 by the English authors is not well enough preserved to allow of their being 1 

 certainly determined to be Lepidodendron, and separated from Sigillaria 

 on grounds which carry conviction. The characters of the latter type 

 will be described in the chapter devoted to it, but it has already been 

 remarked that great caution is necessary in distinguishing between the 

 impressions of certain Lepidodendrae and Sigillariae, because they afford 

 no absolute marks of distinction, with the exception perhaps of the dimen- 

 sions of the scar of separation of the leaf. This cannot be better seen than 



1 Binney (3), f. 32, ff. i, 2. " Williamson (5), p. 341. 3 Hartog (5). 



