THE ANATOMY AND AFFINITY OF PLATYZOMA MICROPHYLLUM. 633 



recorded. In N. Australia both Brown and Gulliver collected this plant by the 

 Gulf of Carpentaria, while Muller found it by the Fitzmaurice River and at 

 Providence Hill. Later on, Martin encountered Platyzoma by the Glenelg River 

 on the north-west coast. In Queensland, both Brown and Hill have found Platy- 

 zoma at Facing Island, while other collectors have discovered it at York Peninsula, 

 Rockingham Bay, and on the downs of the interior. In the Synopsis Filicum it 

 is referred to as inhabiting the tropical and subtropical regions of Australia. In 

 1901, Boodle* laid emphasis on the xerophytic nature of Platyzoma; and in 1910, 

 H. Christ t figured a heterophyllic specimen, and referred to Platyzoma as a 

 xerophytic plant from the desert zones of Australia. 



Regarding the anatomy of Platyzoma very little is known. In 1893, Poirault| 

 briefly described the rhizome, and referred to its curious type of stele. It was here 

 noted that there is a central sclerotic pith, completely surrounded by an endodermis, 

 outside which is a zone of parenchyma surrounded by a broad ring of parenchy- 

 matous xylem. It was further noted that at the periphery of the xylem there is a 

 narrow zone of phloem, which is in turn surrounded by a large-celled pericycle and 

 an endodermis. Special reference was made to the shortening of the internodes, 

 and the crowding of the leaf-traces in the cortex. No leaf-gaps were recorded by 

 Poirault. Nevertheless, he considered this anomalous stelar state to have arisen by 

 obliteration of the leaf-gaps from an original solenostele. We are indebted to 

 Boodle § for fuller information regarding the rhizome. He discussed in greater 

 detail the medullate stele, with its annular xylem and internal endodermis, and 

 described the origin of a leaf-trace by the nipping out of a group of phloem and 

 xylem elements from the periphery of the stele, and indicated the collateral nature 

 of the leaf-trace on its way out through the cortex of the rhizome. He regarded 

 Platyzoma as " a xerophytically reduced form in which the leaf-traces have become 

 small and crowded." He also indicated the possibility of Platyzoma having been 

 derived from a solenostelic form by obliteration of the leaf-gaps and disappearance 

 of the internal phloem of the stele. But, on the other hand, he emphasised the 

 possibility of Platyzoma having been derived from a protostelic Gleichenia, by the 

 formation de novo of a pith and internal endodermis. On the one hand it was noted 

 that the stele in Gleichenia is typically protostelic, and on the other that a widely 

 different type of structure — a solenostelic structure — exists in one species, Gleichenia 

 pectinata. Platyzoma might then be regarded as occupying — as far as stelar con- 

 struction is concerned — a position intermediate between Gleichenia pectinata and all 

 other known Gleichenias. 



To the view that the present stelar condition of Platyzoma has arisen by trans- 

 formation of a solenostele both Jeffrey and Tansley have lent their support, the 



* " Anatomy of the Gleicheniaceae," Ann. Bot., xv. t Die Geographic der Fame, p. 180. 



J " Reclierches sur les Gry-ptogames vasculaires," Ann. des Sciences Nat. Bot., 7 e ser., torn, xviii. 

 § " Anatomy of the Gleicheniacese," Ann. Bot., xv, 1901. 



