6G4 MR R. KIDSTON AND MR D. T. GWYNNE-VAUGHAN ON 



line of descent is at all close to the Osmundaceous, we must be prepared for the 

 existence of a Zygopteris with a solid xylem like that of Thamnopteris, the central 

 elements consisting entirely of short tracheae without any parenchyma. Such a 

 discovery, in fact, we hopefully anticipate. 



To turn now to the mesarchy of the leaf-trace xylem in the neighbourhood of the 

 stem stele. This point, which has already been described as of frequent occurrence in 

 Zalesskya gracilis, becomes in Thamnopteris a perfectly regular and constant pheno- 

 menon. If it can be maintained that the structure of the leaf-trace in its course 

 through the cortex of the stem may give some indication of the earlier stages in its 

 phylogenetic development, this mesarchy becomes a matter of considerable interest. 

 We have already discussed this subject in a short paper published in the Proceedings of 

 this Society,* in which we endeavoured to establish the theory that the adaxially curved 

 endarch xylem of the leaf-trace in the free petiole of the Osmundacese has been 

 gradually evolved from a primitively mesarch strand through a series of changes that 

 are briefly recapitulated by the leaf-trace xylem of Thamnopteris in its passage 

 through the cortex (cf. PL IV.). We also regard it probable that a similar origin may 

 be ascribed to all Filicinean traces that can be derived from the adaxially curved 

 endarch type. 



A peculiar and conspicuous feature of the leaf-trace of Thamnopteris is the appear- 

 ance of mucilage sacs in the pericycle, which afford unexpected confirmation of its close 

 relationship with the modern Osmundacese. As regards Bathypteris and Anomorrhosa, 

 in spite of the absence of all knowledge of their stem structure, the close correspondence 

 of the essential points in the structure of their petioles with those of the other 

 Osmundacese practically ensures their inclusion in the order. Judging from the 

 structure of the petiole only, Bathypteris is by far the most aberrant type. It is 

 separated sharply from all the others, not only by the complete absence of stipules, but 

 also by the presence of spinose emergences and by the occurrence of sclerotic elements 

 in the xylem sheath. The unfortunate absence of the stem renders any speculation 

 as to the primitive or specialised nature of these interesting features too hazardous 

 to be pursued at present.t 



Our thanks are especially due to Mons. Th. Tschernyschew for generously supply- 

 ing us with materials for the examination of these classical fern stems, and also to 

 Mons. Zalessky, through whom they were communicated to us, and for his many other 

 kind offices. 



* Gwynne-Vaughan and Kidston, "On the Origin of the Adaxially Curved Leaf-trace in the Filicales," Proc. 

 Roy. Soc. Edin,, vol. xxviii., part vi. (No. 29), p. 433, 1908. 



t Since the publication of the first part of these memoirs we have been able, through the great kindness of Prof. 

 C. Eg. Bektrand, to examine a type specimen of Grammatojjteris Rigolloti. The specimen exhibited only an opaque 

 polished surface, and accurate conclusions as to its structure were difficult to arrive at. Still, the xylem strands of the 

 leaf-traces in the cortex were almost straight oblong bands with small elements along the margins of their extremities. 

 So far as we could make out, the general features of this fossil prohibit the especially close affinities with the 

 Osmundacew that we previously suspected. 



