2 6o STUDIES IN FOSSIL BOTANY 



doubt their homology. Some authors have regarded 

 the appendages as modified leaves, a view which was 

 primarily suggested by their arrangement on the axis. 

 Recent discoveries have tended somewhat to increase 

 the analogies with leaves ; the so-called transfusion- 

 tissue of the Stigmarian rootlet (see above, p. 253) has 

 been compared with that of the leaf, though the two 

 tissues differ as much in arrangement as they no 

 doubt did in function. Quite recently Professor Weiss 

 has found, in connection with the rootlet-trace of the 

 Stigmarian Xenophyton, a strand of cells analogous to 

 the parichnos of the leaves, but situated on the opposite 

 side of the bundle (next the xylem instead of next the 

 phloem). 1 Such resemblances as these do not count 

 for much in establishing an homology, as Professor Weiss 

 points out. On the other hand, the course of the 

 rootlet-traces in Xenophyton, as already mentioned, is 

 remarkably like that of the leaf-traces in the stems of 

 Lepidodendreae. 



The arguments in favour of the root-nature of the 

 appendages are their dichotomous branching, monarch 

 structure, general resemblance to the root of allied 

 recent Lycopods, and the occurrence of secondary 

 thickening. In favour of their homology with leaves 

 we have their quincuncial arrangement (see Fig. 97), 

 bud-like convergence at the apex, the transfusion-tissue 



Cf. Bruchmann, Selaginella spinulosa, Plate i. Fig. 13, 1897; Farmer, "On 

 Isoetes" Ann. Bot. vol. v. Plate v. Fig. 4, 1S91 ; Scott and Hill, " Structure 

 of Isoetes Hystrix," Ann. Bot. vol. xiv. Plate xxiv. Fig. 30, 1900. For 

 information on the structure of rhizophore and root in many species of Sela- 

 ginella, see HarveyGibson, "Contributions towards a Knowledge of the Ana- 

 tomy of the Genus Selaginella, Part iv. The Root," Ann. Bot. vol. xvi. 1902. 

 1 Weiss, "The Parichnos in the Lepidodendraceae," Mem. and Proc. 

 Manchester Lit. and Phil. Soc. vol. li. 1907. 



