THE FOSSIL OSMUNDACESE. 229 



regarded as a pericycle, and the limit of the whole leaf-trace is marked by a ring of 

 black carbonaceous matter (fig. 28, car.). 



The Boot. 



The roots arise singly or in pairs directly on the stele. Their xylem strands are 

 attached directly to the xylem of the stele in a line with, but below, the point of 

 departure of a leaf-trace xylem. On their way through the cortex they first of all run 

 towards the apex of the stem (fig. 22, ?'.), and then they turn outwards and their 

 course becomes very irregular. Their xylem strands are diarch, and they do not attain 

 a cortex of their own, so far as could be observed in our specimen. 



Locality. — Found in the same collection as Zalesskya gracilis, Eichwald, sp., but 

 without label. The specimen is, however, believed to have come from the same 

 locality. 



Specimen preserved in the Museum of the Institute of Mines, St Petersburg. 



Zalesskya, n.g. 



Fern stems of considerable size. Leaves spirally arranged. Axis occupied by a 

 single central protostele. Xylem forming a stout continuous ring, or solid to the 

 centre. Xylem differentiated into two regions — a peripheral zone of normal tracheae 

 and a more central region of short, wide vesicular elements. Tracheae with multiseriate 

 pits. Phloem well developed, separated from the xylem by a stout xylem sheath. 

 Protophloem and porose layers absent. Stele surrounded by a very wide parenchy- 

 matous cortex ; outer cortex probably sclerotic. Leaf- trace and its xylem strand at 

 first ovoid in transverse section, with a median mesarch protoxylem near the adaxial 

 side ; further out horse-shoe shaped, with the concavity adaxial and with several 

 endarch protoxylems. 



General Considerations. 



The most interesting points brought to light by the investigation of the genus 

 Zalesskya are, first of all, the absolute confirmation of the occurrence of a completely 

 continuous and perfectly protostelic xylem ring in the vascular anatomy of the 

 Osmundacese, and, secondly, the marked distinction that exists between the elements 

 of the peripheral and those of the more central regions of the xylem. The peripheral 

 xylem zone is obviously represented in the recent Osmundacese by the ring of xylem 

 strands that occurs in the stele, but the central xylem, with its short, wide reticulate or 

 porose elements, appears to have been lost in the existing forms. 



As regards Zalesskya diploxylon, at any rate, we feel convinced that the central 

 xylem occupied the whole of the centre of the stele in the living plant. Further, we 

 accept the deduction suggested by this conclusion, that the vascular anatomy of the 

 Osmundacese must he derived from a protostele with a solid central homogeneous 



TRANS. ROY. SOC. EDIN., VOL. XLVI. PART II. (NO. 9). 34 



