That the stelar problem considered in this paper may be more clearly visualised, it will be well to divest of purely theoretical considerations the facts of stelar structure already known forPlatyzoma, and to summarise such structural features as bear on the subject in hand.In the first account ofPlatyzomagiven by Robert Brown in 1810 (l), the habit of the stem and heterophyllic leaves, and the general form and position of the sporangia, were alone described. But in 1832 (2) a rough analysis of

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... the structure of the stem was added, and the tubular nature of its stele was recognised. No material contribution to the knowledge of the stelar structure was made until 1893 (3), when Dr Poirault described the histology of the stele as shown by a small fragment of stem. In this material there was a sclerotic pith completely surrounded by an endodermis, outside which lay a zone of parenchyma surrounded by a broad ring of parenchymatous xylem. At the periphery of the xylem there was a narrow zone of phloëm surrounded by a large-celled pericycle and an outer endodermis. Neither leaf-gaps nor perforations were found in the stele, and accordingly the complete isolation of the pith from the cortex was recognised.