230 MR R. KIDSTON AND MR D. T. GWYNNE-VAUGHAN ON 



xylem mass. In fact, these fossils seem to disclose some of the steps by which the 

 transition actually took place. In Zalesskya diploxyJon the central trachea? have 

 become short and wide, and seem to have changed their water-carrying function for one 

 of water-storing. In the recent and the more advanced forms they have become still 

 further modified, and, losing all trace of their original tracheal nature, they have 

 given rise to a thin- walled parenchymatous pith. From this point of view it is 

 clear that the central ground- tissue of the recent Osmundacese must be regarded as 

 phylogenetically derived by modification from the central xylem of a solid protostele, 

 and that primitively it had no relation or connection ivith the cortex ivhatever. The 

 vascular anatomy of the Lepidodendrese and Sigillarise provides an instructive and 

 closely parallel series of developments. A similar advance is made, starting from a 

 homogeneous solid mass of xylem (L. rhodumnense, etc.), through such a form as 

 L. vasculare, which has a pith intermingled with short isodiametric tracheae, leading on 

 to forms with a pure pith (L. Harcourtii, etc.). The next stage is the partial 

 breaking up of the narrow xylem ring as seen in Sigillaria spmidosa (Scott, Studies 

 in Fossil Botany, p. 200), until finally a stage is reached in which the xylem strands 

 are completely separate from one another, as in Sigillaria Menardi (Brongniart, 

 Observations sur la Structure du Sigillaria elegans, pi. xxv., figs. 3 and 4 ; pi. xxvii., 

 fig. 1). At this point reference should be made to an interesting observation by 

 Seward and Ford in their paper on the anatomy of Todea (Trans. Linn. Soc. Lond., 

 vol. vi., pt. 5, 1903, pp. 248 and 249). They record the occasional occurrence of short, 

 wide tracheae with reticulate thickenings at the inner margins of the xylem strands of 

 Todea superba and T. hymenophylloides. The former is figured on pi. xxix., figs. 

 30 and 31. These elements may perhaps be regarded as the vestigial remains of the 

 ancestral central xylem. 



The peripheral tissues of the stele in the living Osmundacese possess some special 

 characters of exceptional interest and peculiar to the order — in particular, the absence 

 of a true protophloem and the presence of the so-called " porose layers " on the outside 

 of the metaphloem. In Zalesskya both these tissues are absent, and the large sieve- 

 tubes of the metaphloem abut directly upon a zone of parenchyma that we have called 

 the pericycle. It appears, therefore, that the porose layers are a relatively late develop- 

 ment, and that Seward and Ford are right (I.e., p. 242) in regarding it as derived from 

 the pericycle. In the Osmundaceous affinity the phloem of the stem has never become 

 differentiated into protophloem and metaphloem, not even in the most advanced forms. 

 On the other hand, a well-developed protophloem is present in the leaf-trace of the 

 recent genera and also in Osmundites skidegatensis. 



As regards the zone of tissue that we have called the pericycle in these two fossils, 

 it only deserves the name in virtue of its position. No real delimitation is possible 

 between its cells and those of the inner cortex, and there is no definite layer that can 

 be identified as an endodermis. In fact, the state of affairs in these primitive forms would 

 be best expressed by saying that the phloem of the stele was surrounded by a narrow 



