THE FOSSIL OSMUNDACE^E. 231 



sheath formed by slightly specialised cells of the inner cortex. An endodermis might 

 have been demonstrable in the living plant as in the modern genera, and whenever it is 

 present it must have been differentiated from the outer cells of this sheath, which may 

 account for the slight irregularities which it frequently shows in the recent forms. These 

 observations on the phloem and pericycle are, of course, far too limited to do more than 

 provide mere suggestions as to the origin of these tissues. A complete knowledge of 

 this region of the stele in other primitive members of the order is necessary before 

 any confident conclusions can be reached. 



Owing to the decorticated nature of our specimens, it is unfortunately impossible to 

 say whether the base of the petiole possessed stipular wings or not, and no comparisons 

 can be made with the structure of the petiole in the other members of the Osmundacese. 

 It should be mentioned, however, that in Zalesshya there is no alternation of scale- 

 leaves and foliage-leaves such as occurs in the recent genera and in the Jurassic 

 representatives Osmundites Dunlopi and 0. Gibbiana. The mesarch position of the 

 protoxylem in the leaf-trace when still in close proximity to the stem stele is, we believe, 

 a point of considerable importance ; but since another Osmundaceous fossil to be described 

 in the next part of this paper promises to throw some light upon the point, it will be 

 best to leave it over for future consideration.* 



DESCRIPTION OF FIGURES. 



Figs. 8 a-f are from drawings ; all the others are from untouched photographs. 



The following lettering is used throughout: xy., xylem; prx., protoxylem; ph., phloem; S.T., sieve- 

 tubes; per., pericycle; C, cortex; l.t., leaf-trace; r., root. 



Plate I. 



Fig. 1. Zalesshya gracilis. External surface of the whole fossil. Nat. size. (Photograph by Mons. R. 

 Koch; forwarded by Mons. M. Zalessky.) 



Fig. 2. Znlesskya gracilis. A complete transverse section, st., stele; Sc. C, sclerotic cortex; i. C, 

 parenchymatous cortex ; A., tract of inner cortex accompanying the leaf-trace through the sclerotic outer 

 cortex. x about 2J. (Slide 1253.) 



Fig. 3. Zalesshya gracilis. Portion of the vascular ring in transverse section, o. xy., outer xylem : 

 i. xy., inner xylem. The black band at x represents crushed tracheides. x 30. (Slide 1253.) 



Plate II. 



Fig. 4. Zalesshya gracilis. The vascular ring in longitudinal section, xy. sh., xylem sheath. x 30. 

 (Slide 1258.) 



Fig. 5. Zalesshya gracilis. Surface views of the tracheal walls as seen in longitudinal section, a, a 

 single vertical series of pits ; b, two such series ; c, two series widely separated ; d, three series, the median 

 one being smaller than the lateral ; e, three series all similar ; /, four equal series ; g, reticulate or porose 

 pitting; h, terminal wall between two reticulate or porose elements. x 250. (Slides Nos. 1255, 1256, 

 1257, 1258.) 



* We beg to acknowledge our indebtedness to the Executive Committee of the Carnegie Trust for a grant to 

 defray the cost of the plates illustrating Parts I. and II. of this paper. 



