468 R. KIDSTON AND D. T. GWYNNE-VAUGHAN ON 



The bitter method, of course, presumes the existence of tin intermediate stage in which 

 the pith consists of tracheae mixed with parenchyma. Osmundites Kolbei at once 

 determines our choice between these two alternatives by pronouncing so clearly as it 

 does in favour of the latter. This decision once made inevitably leads us to regard 

 the mixed pith of the Zygopteridese as being of the same nature and origin as that of 

 the Osmundacesp,, especially so since we believe that the two orders have descended 

 from a common ancestor. In Zygopteris Grayi and Z. corrvgata, however, the central 

 area that has undergone the change also extends outwards in the form of arms or rays 

 opposite the points of attachment of the departures until it has reached their decurrent 

 protoxylems. In the lower part of their course, at any rate, these protoxylems abut 

 directly on to the pith. They have no longer any continuous centripetal xylem on 

 their inner faces, and may therefore be regarded as endarch.* An Osmundaceous stele 

 with a continuous ring of xylem and a mixed pith corresponding exactly to that of 

 Zygopteris Grayi has not yet been found ; if it did exist, the difference between it and 

 the stele of Zygopteris Grayi would lie mainly in the star-like outline of the latter. 

 This, of course, is only a very superficial point, due entirely to the early and excessive 

 prominence of the xylem of the departures. In Zygopteris corrugata even this 

 difference is much less marked, and, since the leaves arise at a fair distance apart, its 

 stele makes a close approximation to the hypothetical Osmundaceous stele mentioned 

 above. Indeed, the resemblance between the stele of Zygopteris corrugata and a 

 primitive type of Osmundaceous stele has already been commented on by TANSLEY.t 



If the parallelism in stelar development in the Osmundacese and Zygopteridese is 

 at all close we must anticipate the discovery of a primitive form of Zygopterid stele 

 with a solid xylem mass, the central tracheae of which are short and transitional towards 

 a pith, as in Zalesskya and TJiamnopter^is. 



Shortly after we had arrived at this conclusion we were shown a stele exactly of this 

 type by Mr W. T. Gordon in a block of Burntisland material that also contained a 

 large number of petioles of Zygopteris (Diplolabis) Romeri. We have followed Mr 

 Gordon's painstaking and persistent work on this material with the greatest interest, and 

 are delighted with his richly deserved success in establishing the organic continuity of 

 this stele with the petioles in question.^ He has kindly allowed us to figure some of 

 his preparations for the purpose of this paper, and fig. 29 is a photograph of this very 



* Incidentally this conception of a direct, in situ, metamorphosis of primary xylem into pith, if carried a point 

 further, may throw some light upon the unusual structure of the primary xylem of the stems of Pitys antiqua and 

 Calamopitys fascicularis as described by Dr Scott (Trans. Roy. Soc. Edin., vol. xl., part ii. (No. 17), p. 331, 1902). 

 These stems possess a pith surrounded by a large amount of secondary xylem. A certain amount of primary xylem 

 is also present in the form of isolated strands with mesarch protoxylems situated in the pith at a considerable distance 

 from the inner margin of the secondary xylem. 



It is possible that this may be accounted for by supposing that the protoxylems were originally immersed in a 

 continuous zone of xylem, and that not only has the metaxylem lying between them become converted into pith, but 

 also the greater part of that lying to the outside of them. So that all that is left of it is a few elements in immediate 

 relation to the protoxylem strands themselves, which remain in their original immersed positions, and are now sur- 

 rounded by pith instead of metaxylem. 



t The Evolution of the Filicinean Vascular System, p. 105, 1908. 



\ Gordon. Described in paper read at Roy. Soc. Edin., 20th Dec. 1909. 



