444 STUDIES IN FOSSIL BOTANY 



secondary growth, depending on the peculiar distribu- 

 tion of the cambium ; * the whole structure is no doubt 

 an adaptation to a climbing habit. In Medullosa the 

 primary organisation was evidently polystelic (dialy- 

 stelic), as in most recent Ferns, and there is no reason 

 to assume that it had any relation to scandent growth.' 



It may be pointed out here that the polystely of 

 Medullosa differs from that of the typical polystelic 

 or dictyostelic Ferns in having, so far as appears, no 

 relation to the presence of " leaf-gaps." In the species 

 at present investigated the leaf-traces of Medullosa are 

 given off from the outer faces of the steles, and leave 

 no gaps ; the breaking up of the original stele depended 

 on some other cause. Further investigation is necessary 

 to show whether this distinction is absolute. 



In one or two of the Continental Medulloseae, 

 something has been ascertained as to the course of the 

 leaf-traces ; thus, in M. stellata, the leaf-traces are given 

 off from the peripheral steles, and each is at first 

 surrounded by its own zone of secondary wood. They 

 then divide up, lose their secondary tissues, and become 

 collateral, just as in M. anglica. They have not, how- 

 ever, been traced into the leaves, which are as yet 

 unknown in M. stellata. 



In another Continental species, M. Leuckarti? the 

 leaf- traces behave in the same way, though their 

 connection with the steles appears not to have been 

 seen at present. In M. Leuckarti the structure of the 

 leaf-bases is known, and, as in the English species, is 

 of the type of " Myeloxylon Landriotii." 



1 Cf. De Bary, Comparative Anatomy of Phanerogams and Ferns, 

 English edition, 1884, p. 581. 



2 Solms-Laubach, " Ueber Medullosa Leuckarti," Bot. Zeitung, 1897. 



