68 THE MORPHOLOGY OF PTERIDOPHYTES 



Lepidodendrales 



The Lepidodendrales, over 200 species of which are known, 

 first appeared in Lower Carboniferous times and reached 

 their greatest development in the Upper Carboniferous 

 swamp forests, in which members of the Lepidodendraceae, 

 Bothrodendraceae and Sigillariaceae were co-dominant with 

 the Calamitales and formed forests of trees 40 m or more in 

 height. The fourth family, Pleuromeiaceae, is represented 

 by a much smaller plant, Pleuromeia, from Triassic rocks, 

 and approached more nearly to the modern Isoetales. The 

 Carboniferous genera had stout trunks, some with a crown 

 of branches, others hardly branching at all, but all possessed 

 the same type of underground organs, known collectively as 

 Stigmarian axes. Some species of Lepidodendron, (e.g. L. 

 obovatum. Fig. 12A) showed very regular dichotomies in its 

 crown of branches, but others approximated to a mono- 

 podial arrangement because of successive unequal dicho- 

 tomies. While the trunks and branches of all species of 

 Lepidodendron and Lepidophloios were protosteHc and 

 exarch, there was nevertheless considerable variation in 

 stelar anatomy, from species to species, and from place to 

 place within one individual. Some species had soHd proto- 

 steles, others meduUated protosteles; some had abundant 

 secondary wood produced by a vascular cambium, some had 

 little and others had none at all; in some, the stele of the 

 trunk had secondary wood, while that of the branches 

 lacked it altogether. Thus, Lepidodendron pettycurense and 

 L. Rhodumnense (both Lower Carboniferous species) had 

 soUd protosteles, the former having secondary wood in 

 addition, but the latter being without it. Lepidodendron 

 selaginoides ( = L, vasculare), from the Coal Measures, pro- 

 vides an interesting case of partial meduUation, for the 

 central region of the axis consisted of a mixture of parenchy- 

 ma and tracheids, round which was a solid ring of tracheids. 

 The secondary wood of this species was often excentric in 

 its development, as illustrated in Fig. iiB. 



