tl6 THE MORPHOLOGY OF PTERIDOPHYTES 



Stauropteridaceae* Stauropteris* 

 Botryopteridaceae * Bo try op teris * 



The Cladoxylales are a particularly interesting group, 

 whose correct phylogeny has long been a matter of con- 

 troversy. On the one hand, they show a number of features 

 in common with the Psilophytales and, indeed, Pseudo- 

 sporochnus has only recently^® been transferred from that 

 group. On the other hand, they show features in common 

 with the Coenopteridales, whose later representatives had 

 already begun to look fern-hke before they became extinct. 

 The group thus stands in an intermediate position which 

 strongly suggests a genuine phylogenetic connection be- 

 tween the two groups. 



Several species of Cladoxylon are known, of which the 

 earhest is C. scoparium, and our knowledge of this is based 

 on one specimen about 20 cm long from Middle Devonian 

 rocks of Germany. According to the reconstruction of the 

 plant by Kraiisel and Weyland^^ (Fig. 17 A), there was a 

 main stem, about 1-5 cm in diameter, which branched rather 

 irregularly. Some of the branches bore fan-shaped leaves 

 (Fig. 17B) ranging in size from 5 mm to 18 mm long. Some 

 leaves were much more deeply divided than others, but all 

 showed a series of dichotomies. On some of the branches, 

 the leaves were replaced by fertile appendages which were 

 also fan-shaped, each segment terminating in a single 

 sporangium (Fig. 17C). 



The vascular system was highly complex and was poly- 

 stelic; each of the separate steles was deeply flanged; both 

 scalariform and pitted tracheids were present in the xylem. 

 Such complex vascular structure is characteristic of all the 

 species of Cladoxylon and some had the additional comph- 

 cation of secondary thickening. C. radiatum was similar to 

 C scoparium in that all the xylem was primary, and Fig. 

 17D illustrates the way in which several xylem flanges were 

 involved in the origin of a branch trace system. It also 



