142 THE MORPHOLOGY OF PTERIDOPHYTES 



of phloem (and, presumably, endodermis) round the out- 

 side. The term 'dictyoxyHc stele' can conveniently be used 

 to describe this arrangement. Poor preservation does not 

 allow any statement to be made about the central pith 

 regions of these two forms, but in the Lower Cretaceous 

 O. Kolbei there was definitely a mixed pith. The Cretaceous 

 species O. skidegatensis had a pith of pure parenchyma and 

 showed a further advance in having some internal phloem, 

 while O. Carnieri was the most advanced of all, in being 

 truly dictyostehc. This is most interesting, for it is a con- 

 dition not achieved by any modern representatives of the 

 group. Most of these are no further advanced in stelar 

 anatomy than the Jurassic Osmundites Gibbeana. 



Of the living genera, Osmunda (fourteen species) is wide- 

 spread in both hemispheres, Leptopteris (six species) is con- 

 fined to Australasia and the South Sea Islands, while Todea 

 is represented by the single species T. barbara, found in 

 S. Africa and Australasia. (Some taxonomists include 

 Leptopteris in the genus Todea.) Only one species, Osmunda 

 regalis — the 'Royal fern' — is represented in the British flora. 

 Its stems are massive and branch dichotomously to form 

 large hummocks. Todea barbara may have a free-standing 

 trunk I m or more high, and so also may Leptopteris 

 hymenophylloides, while one species of Leptopteris from 

 New Caledonia attains a height of 3 m. 



A transverse section of the stem of a mature Todea (Fig. 

 21K) exhibits a typical dictyoxylic condition. The central 

 medulla is surrounded by separate blocks of xylem, outside 

 which there is phloem and a continuous endodermis. 

 Occasionally, some internal phloem occurs, but no internal 

 endodermis. Most species of Osmunda are similar, but O. 

 cinnamomea sometimes has an internal, as well as an 

 external, endodermis (Fig. 21B). The types of xylem element 

 present are similar, in some respects, to those of the 

 Marattiaceae,^^ and the position of the protoxylem ranges 

 from endarch in Todea to nearly exarch in Osmunda. 



