98 THE MORPHOLOGY OF PTERIDOPHYTES 



were unable to support their own weight and must have been 

 prostrate on the ground, or must have depended on other 

 plants for support. In general appearance, they probably 

 looked rather hke a Galium ('Bedstraw'). The anatomy of 

 the stem was most peculiar in its resemblance to that of a 

 root, for in the centre was a triangular region of solid prim- 

 ary wood, with the protoxylems at the three corners in an 

 exarch position. In the Lower Carboniferous species, S, 

 insigne, the protoxylem tended to break down to form a 

 'carinal' canal, but in the Upper Carboniferous species this 

 rarely happened. Outside the primary wood, a vascular 

 cambium gave rise to secondary wood, first between the 

 protoxylems, and then later extending all round. However, 

 the wood opposite the protoxylems was composed of smaller 

 cells than on the intermediate radii, resulting in a pattern 

 which is quite characteristic and which is recognizable at a 

 glance in transverse sections (Fig. 15L). The primary wood 

 consisted entirely of tracheids (i.e. without any admixture of 

 parenchyma) and they bore multiseriate bordered pits on 

 their lateral walls. The tracheids of the secondary wood also 

 bore multiseriate pits, but they were restricted to the radial 

 walls. Between the tracheids, there were wood rays. These 

 were continuous in S. insigne, but were interrupted in S. 

 plurifoliatum where they were represented only by a group 

 of parenchyma cells in the angles between adjacent tracheids. 

 Large stems had a considerable thickness of cork on the 

 outside, formed from a deep-seated phellogen. 



The leaves of Sphenophyllum showed a wide range of 

 structure, some being deeply cleft, while others were entire 

 and deltoid (Figs. 15M-O); yet all received a single vascular 

 bundle, which dichotomized very regularly within the 

 lamina. Some species were markedly heterophyllous, as 

 illustrated in Fig. 15K, and in these the deeply cleft leaves 

 were usually near the base, while the entire ones were higher 

 up on lateral branches, an arrangement that suggests that 

 the former might represent juvenile fohage. 



