634 MR JOHN M'LEAN THOMPSON ON 



latter stating definitely * that " Platyzoma, with its extreme xerophily and anomalous 

 leaf-traces, may almost certainly be regarded as reduced from the solenostelic type." 

 Bower and others have inclined to the belief that the original condition of Platyzoma 

 was protostely. But in any case, Boodle considered that the construction of the 

 stele of Platyzoma was sufficiently individualistic to justify the separation of Platy- 

 zoma from Gleichenia, as proposed by Hooker and Baker. It is then evident that 

 the facts advanced regarding the stele have led neither to the removal of Platyzoma 

 from the Gleicheniaceae nor to a definite settlement of its position within this group. 



A brief consideration of the characters which have been employed in determining 

 Platyzoma s systematic position may not be out of place at this point. The habit 

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

 to place Platyzoma in the Gleicheniaceae. These same general features have likewise 

 been the main guides of later systematists. But at the present day, when convergent 

 evolution is being widely recognised among Ferns, it is necessary, for systematic 

 determinations, to be provided with detailed knowledge of the anatomy of stem, leaf, 

 and root, and of the original position and form of the sporangia. The exact position 

 and form of the annulus and the form and output of spores per sporangium are of 

 much importance. The nature of the dermal appendages should also be determined. 

 In fact, it has been found that while general habit has frequently proved a true index 

 in phyletic groupings, at other times — when taken by itself— it has led to gross 

 error. In the case of Platyzoma no definite information has hitherto been advanced 

 regarding the anatomy of leaf and root, nor has any accurate account been given of 

 the form and development of the sporangia, the character of the annulus, and the 

 spore form and output. 



Both Poirault and Boodle have contributed to our knowledge of the stele of 

 Platyzoma. Nevertheless, a wider knowledge on this subject is necessary, and the 

 facts already advanced are far from sufficient to prove or disprove the proposed 

 Gleicheniaceous affinity. Further, one would have expected that the heterophylly 

 noted by Brown, and shown in Christ's figure, was a character of too great im- 

 portance to be neglected by the systematists. Boodle has informed me that in five 

 of the sheets of specimens of Platyzoma in Kew Herbarium the heterophylly is clearly 

 shown ; while in a beautiful specimen, collected in 1902 by Professor Baldwin 

 Spencer at Boorroololoo, N.T., and kindly supplied to us by the authorities of the 

 National Herbarium of New South Wales, Sydney, this heterophylly is a conspicuous 

 feature (figs. 6 and 8). 



As to the original distinction drawn by Brown between the branched leaves of 

 Gleichenia and the unbranched leaves of Platyzoma a few remarks are necessary. 

 It was noted by Bower in 1899 t that the pinnate leaf of Platyzoma is occasionally 

 forked, as is seen in specimens of a " forma furcata" sent by the late Baron Sir F. V. 



* " Lectures on the Evolution of the Filicean Vascular System," New Phytologist, 1908. 



t "Studies in the Morphology of Spore-producing Members," Phil. Trans. Royal Soc. London, iv. 



