44S COENOPTKRIDEAE [CH. 



of the rachis occurs in the plane of the frond, that is in the 

 plane represented by the horizontal arm of xylem in Zygopteris 

 primaria connecting the two antennae or cross-pieces. In the 

 Zygoptereae the branches from the petiole vascular axis lie in 

 a plane at right angles to that of the frond; they lie in the 

 transverse and not in the horizontal plane. The two strands 

 shown in fig. 308, B, 4, have been formed by the division of a 

 single strand, 3, in the transverse plane (i.e. in the plane of the 

 paper). As Tansley' points out, a type of branching super- 

 ficially similar to, though not identical with this, is seen in 

 some recent species of Oleichenia and Lygodium. In this 

 connexion it is worthy of note that a fern figured by Unger 

 from Thuringia as Sphenopteris petiolata Goepp ^ bears pinnae 

 in two rows on the rachis whjch are characterised by repeated 

 branching and by a very narrow lamina or by slender naked 

 axes; the occurrence of this form of frond in rocks con- 

 taining Clepsydropsis antiqua (fig. 308, A) suggests a possible 

 connexion between the petrified rachis and the impressions of 

 the leaves. 



The vascular strand of the rachis of Zygopteris primaria 

 (fig. 308, D) is simpler than that of most of the Zygoptereae 

 and exhibits a close resemblance to the type of strand described 

 by Renault as Diplolabis (fig. 308, C). 



Diplolahis. 



Renault^ instituted this genus for two species from the 

 Culm beds and Coal-Measures of France based on the structure 

 of the petioles. The stems are unknown. The main rachis 

 has a stele similar to that of Zygopteris primaria, but dis- 

 tinguished by its greater similarity, in transverse section, to 

 an X rather than to the letter H : the long transverse bar in 

 Zygopteris is here much reduced in size. The petiole of 

 Diplolahis forensis Ren. (fig. 308, C) has a diameter of 



> Tansley (08) p. 22. ^ Unger and Bichter (56) PI. vi. fig. 19. 



3 Benault (96) A. p. H. 



•" The Diplolabis type of strand is very similar in the form of the inetaxylem 

 to the conducting strand of a lateral vein in Scolopendrium officinarum [of. 

 Pelourde (09) fig. 3, p. 117]. 



