XXV] ANKYROPTERIS, ETC. 471 



distinction between leaf and stem steles is still more pronounced. 

 It is perhaps legitimate to regard these types as representing 

 an ascending series, the more primitive of which are distin- 

 guished by the greater similarity between leaf and stem, 

 organs diiferentiated from a primitive thallusS that is from a 

 vegetative body. Portions of this ultimately became specialised 

 as lateral members or leaves, while a portion acquired the 

 character of a radially constructed supporting axis or stem. 

 The vascular strand characteristic of the Zygoptereae is 

 represented by the H -shaped form as seen in Ankyropteris 

 corrugata or in a more complex form in A. bibractensis. This 

 style of strand may be regarded as a development from the 

 simple strands of Orammatopteris and Tuhicaulis or Botryo- 

 pteris antiqua along other lines than those followed by 

 B. forensis. The extension of the xylem in two symmetrically 

 placed arms at the ends of the cross-piece of the H is correlated 

 with the habit of branching of the leaf-system which forms one 

 of the striking peculiarities of many of the Zygoptereae. The 

 solid type of stele characteristic of the Botryoptereae is closely 

 matched by that in the Lower Carboniferous stem discovered 

 by Mr Gordon''. By the partial transformation of the central 

 xylem region into parenchymatous tissue and the concentra- 

 tion of water-conducting elements in the peripheral region the 

 style of Ankyropteris corrugata was developed. The vascular 

 strand of the older plant, which is of the Diplolahis type, may 

 be regarded as a more primitive style than that of the H-form 

 of petiole strand represented by Ankyropteris corrugata. A 

 further stage in evolution is seen in the stem stele of Anky- 

 ropteris Grayi and A. scandens, both of which have the H-form 

 of meristele. This step in increasing complexity of stem stele, 

 though probably connected with the increasing specialisation of 

 the leaf-traces, as held by Mr Tansley, may also be associated 

 with the development of a climbing habit. In Asterochlaena 

 laxa Stenzel (fig. 324) and A. ratnosa (Cotta)' the tendency 

 towards a stellate expansion of the originally cylindrical form 



1 Tansley (08). ^ See p. 447. 



3 Stenzel (89) p. 15, Pis. ni. iv. 



