LYCOPSIDA 75 



broad rays (through which the rootlet traces passed) which 

 divided the wood into very characteristic wedge-shaped 

 blocks. 



The true nature of Stigmarian axes has long been a 

 problem to morphologists, for although doubtless they per- 

 formed the functions attributed to roots in higher plants 

 (absorption and anchorage), yet they were different in so 

 many respects from true roots and, at the same time, were 

 so different from the aerial axes that they appear to have 

 belonged to a category of plant organization that was quite 

 unique. Even the nature of the 'rootlets' is open to question, 

 for specimens of Stigmarian axes are known which bore 

 leaf-like appendages instead of rootlets. Once more one is 

 forced to the conclusion that the categories root, stem and 

 leaf have no clear distinction at the lower levels of evolution. 



Pleuromeia (Fig. 13 A) was a much smaller plant than the 

 other members of the Lepidodendrales, for its erect un- 

 branched stems were Httle more than i m high and 10 cm in 

 diameter. The lower parts of the stem were covered with 

 spirally arranged leaf scars, while the upper parts bore 

 narrow pointed ligulate leaves about 10 cm long, attached 

 by a broad base. The plant was heterosporous and dioecious, 

 and the sporangia were borne in a terminal cone made up of 

 numerous spirally arranged circular or reniform sporophylls. 

 Although early descriptions described the sporangia as on 

 the abaxial side of the sporophyll, most morphologists 

 believe this to be an error and it is usually accepted that, as 

 in all other lycopods, they were on the adaxial side. Verifica- 

 tion of this must await the discovery of better preserved 

 specimens, for no petrified material has yet been found. For 

 this reason, also, httle is known of the internal anatomy of 

 the plant. 



Below the ground, Pleuromeia was strikingly different 

 from the other members of the Lepidodendrales, for, in- 

 stead of having spreading rhizomorphs of the Stigmaria 

 type, it terminated in four (or sometimes more) blunt lobes. 



