122 THE MORPHOLOGY OF PTERIDOPHYTES 



Other species the petiolar trace was much less extreme, 

 having less 'waist', but all were ahke in that the islands of 

 parenchyma were much extended tangentially and in that 

 the peripheral loop remained closed throughout the origin 

 of a pinna trace. Another pecuhar feature was that the pinna 

 trace was undivided, suggesting that the pinnae were single, 

 instead of paired as in most other members of the group. A. 

 Grayi, from British coal measures, had a stem of considerable 

 length which was over 2 cm in diameter. It was probably a 

 climbing plant. The petioles were borne in a 2/5 phyllotactic 

 spiral, corresponding with the five rays of the stellate stele 

 (Fig. 17N). As in other members of the group, there were 

 two distinct regions in the stele, but the inner region showed 

 a clear distinction between a zone of tracheids and a 

 central pith, while the outer region showed no evidence of 

 radial arrangement at all and was clearly primary in origin. 

 The petiolar traces of Etapteris were pecuhar in that the 

 'peripheral loops' remained open throughout (i.e. there were 

 no loops at all). Two pinna traces became detached, fused 

 and then separated again, before passing out into the paired 

 pinnae (Fig. 17P). The Permian species, E. Lacattei, is inter- 

 esting in having progressed further than other members of 

 the group in the evolution of a photosynthetic lamina, for 

 the ultimate pinnules were flattened (Fig. 17Q). In the fertile 

 regions of the frond (Fig. 17R) the pinnules were replaced 

 by groups of sporangia. These were club-shaped, slightly 

 curved, and had a distinct broad annulus of thickened cells 

 (Fig. 17S). Some Etapteris fronds were attached to trailing 

 stems, while others belonged to tree-ferns with stout trunks. 

 The nomenclature of the latter is, however, rather trouble- 

 some. The names Zygopteris and Botrychioxylon which have 

 been used are probably synonymous. Z. primaria had a 

 trunk about 20 cm in diameter, most of which consisted of a 

 tangle of rootlets and leaf bases. In the centre was a single 

 stem 1-5 cm across, with a five-rayed stele showing the usual 

 two regions, but in this case the outer region looks very much 



