GENERA OF BOTRYOPTERIS GROUP 335 



to have been wholly or chiefly centripetal in de- 

 velopment. The petioles, which increased rapidly in 

 diameter after leaving the stem, contain a horse-shoe 

 bundle, with the concavity outwards ; the protoxylem 

 groups clearly lie on the convex, adaxial side. The 

 numerous adventitious roots are diarch. Small spor- 

 angia, much like those attributed to the British species 

 of Botryopteris (Fig. 125) are associated with the 

 specimen, and as no other plant is present in the 

 nodule, this association has some significance. 



The three forms last described, Rachiopteris cylindrica, 

 Grammatopteris, and Tubicaulis, are evidently closely 

 allied to Botryopteris, and, together with that genus, 

 constitute a sub -family, with which the Zygopteris 

 group, including Anachoropteris and Asterochlaena, may 

 be contrasted. The great distinction lies in the stele, 

 a simple cylindrical column with solid xylem in the 

 Botryoptereae, while in the Zygoptereae it has usually a 

 more complex form, and appears always to have a double 

 xylem, with the small internal tracheae accompanied 

 by parenchyma. There is no constant distinction in 

 the petiolar bundle, though here also the Zygoptereae 

 tend to complexity, but the double series of rachis- 

 branches appear to be characteristic of the Zygopteris 

 group. There can be no doubt that the latter are the 

 more highly organised, and the Botryoptereae pre- 

 sumably, although not certainly, the more primitive. 

 It must be remembered that Asterochlaena, which 

 clearly falls into the Zygopteris division, is, on present 

 evidence, the most ancient of the whole family. The 

 two groups have so much in common (especially in the 



