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XX.— The Anatomy and Affinity of Platyzoma microphyllum, R. Br. By John 

 M'Lean Thompson, M.A., B.Sc, Chief Assistant in Botany and Robert 

 Donaldson Research Scholar, Glasgow University. Communicated by Professor 

 Bower, F.R.S. (With Four Plates.) 



(MS. received February 7, 1916. Read February 7, 1916. Issued separately November 6, 1916.) 



In the year 1810 there appeared in Robert Brown's Prodromus Florae 

 Novas Hollandiae et Insulae Van Diemen, p. 160, a brief description of a rare 

 Australian Fern to which the generic name Platyzoma was given. The habit of 

 stem and leaf, and in general the form and position of the sporangia, led Brown 

 to the conclusion that Platyzoma was of Gleicheniaceous affinity. It was noted 

 that the plant was heterophyllic ; for not only did it possess pinnate leaves 

 suggestive of those of certain Gleichenioid forms, but also small, compressed, and 

 filiform leaves devoid of pinnae. To the form of the larger and pinnate leaves 

 Brown paid particular attention, noting chiefly that the leaves of Platyzoma 

 were unbranched, whereas a branched condition was typical of the leaves of 

 Gleichenias. This unbranched condition of the leaf seemed to Brown a suffici- 

 ently distinctive feature to justify the foundation of a new Gleicheniaceous genus 

 — the monotypic genus Platyzoma ; and he named the single species Platyzoma 

 microphyllum. 



Brown's opinion was, then, that Platyzoma was allied to Gleichenia, but 

 was, at the same time, worthy of recognition as a distinct genus. It would 

 appear, however, that at a later date Brown doubted the value of the character 

 on which he had laid most emphasis in founding the genus Platyzoma, for in 

 1814 # he wrote : " Among the Australian Ferns there is no genus absolutely 

 confined to that country, except Platyzoma, but this, perhaps, ought not to be 

 separated from Gleichenia." Still later, in 1838, while describing Polypodium 

 {Dipteris) Horsfiehlii,~\ Brown referred to the axis of Platyzoma, and indicated 

 that it differed anatomically from the axis of all Gleichenias he had examined. 



The pinnate leaves and sporangia of Platyzoma were figured in 1842 by 

 Bauer in Sir Wm. Hooker's Genera Filicum, and here it was affirmed that 

 " Platyzoma differs from Gleichenia more by its undivided leaves, than by any 

 differences in their sporangia." In the Synopsis Filicum, 1874, 1883 (Hooker and 

 Baker), Platyzoma was placed as the first genus of the Gleicheniaceae, and a 

 note was appended affirming that Platyzoma is "a genus which has been too 



* General Remarks, Geographical and Systematic, on the Botany of Terra Australiensis : Misc. Bot. Works of 

 Robert Brovm, vol. i, p. 59, Ray Society Publications, 1866. 

 t Plantse Javanicse. Rariores, p. 2. 

 TRANS. ROY. SOU. EDIN., VOL. LI, PART III (NO. 20). 93 



