THE ANATOMY AND AFFINITY OF PLATYZOMA MICROPHYLLUM. 641 



chymatous layers, and beyond this is an outer endodermis separating the stele from 

 the cortex. 



This curious stele has been interpreted by various observers as a reduced soleno- 

 stele which has lost its inner phloem. But while the leaves are numerous, and many 

 of them are relatively large and are provided with strong leaf-traces from the stele, 

 no leaf-gaps involving a breaking of the continuity of the stele occur at any point. 

 As will be shown directly, the outer endodermis is in no case even temporarily 

 incomplete when a trace is being liberated. A break in the endodermis does cer- 

 tainly arise, but the cortex cannot invade the stele. Further, the xylem of the stele 

 may be so depleted by the demands of a leaf-trace that at the point of departure of 

 the trace the xylem-ring may be broken or traversed by a sheet or mass of parenchyma. 

 But at no point along the entire length of stele examined was the inner endodermis 

 found to be defective. It was neither broken nor did it show any tendency to break, 

 opposite any leaf-trace or at any other point. Poirault has attempted to explain 

 this stele as a reduction from a solenostele from which leaf-gaps have been obliterated. 

 But it may be remarked that not only are leaf-gaps non-existent in the stele of 

 Platyzoma, but no anatomical evidence can be advanced in support of the suggestion 

 that they have existed at any time. There is no connection whatever, either by leaf- 

 gaps or perforations, between the cortex and the pith, and the pith is accordingly 

 recognised as a completely isolated mass of sclerenchyma, bounded on all sides by an 

 unbroken endodermis. But if no evidence can be advanced in support of Poiratjlt's 

 suggestion of a solenostelic origin for the stele of Platyzoma, a serious objection has 

 presented itself to Jeffrey's theory of the origin of the pith in Pteridophytes. 

 Jeffrey has maintained that the pith is, in all cases, of cortical origin, and that it 

 has arisen through an invasion of the stele by the cortical parenchyma at the points 

 of departure of leaf-traces, or at other points. It seems more than difficult to explain 

 the origin of the bulky pith of Platyzoma by such a theory, since neither leaf-gaps 

 nor perforations, nor any tendency towards the formation of such gaps and perfora- 

 tions, have been found in the stele. And further, even supposing the inner endo- 

 dermis could be shown to be locally imperfect or interrupted — so that parenchymatous 

 connections could exist between the cortex and pith — the stele of Platyzoma would 

 still be open to interpretation as of directly protostelic origin. For if the G-leicheni- 

 aceous affinity of Platyzoma could be definitely established, its stele might be 

 interpreted as a transformed protostele, in which sclerenchymatous replacement of 

 the xylem core has taken place, and an inner endodermis has arisen, de novo, 

 around the pith. With one exception — namely, Gleichenia pectinata — the genus 

 Gleichenia is protostelic. The core of the typical Gleicheniaceous protostele is a 

 parenchymatous mass of large storage tracheides ; and if the stele of Platyzoma 

 has been originally protostelic, the inner parenchymatous xylem-cylinder — which 

 has been described as the most variable part of the stele — might be considered the 

 last remnant of a solid xylem core. Lang has shown recently that endodermis can 



