SPHENOPHYLLEAE 97 



of S. insigne with Sphenophyllum are now clearly estab- 

 lished, it is possible that its anatomical characteristics 

 may ultimately justify its generic separation. 



Now that we have dealt sufficiently with the 

 morphology and anatomy of the stem, the organisa- 

 tion of the leaves and roots remains to be considered, 

 before going on to the organs of fructification. I have 

 already mentioned that the leaves of Sphenophyllum are 

 whorled, and that they vary very much in form, some 

 being broad and wedge-shaped (the type of leaf from 

 which the genus derives its name), while others are 

 deeply divided, or even assume a very simple linear 

 form. It appears to be the rule, that the number of 

 leaves in a whorl was always some multiple of three, 

 a fact which is no doubt correlated with the triarch or 

 hexarch structure of the vascular cylinder. Six was 

 perhaps the most usual number (see Fig. 36, B) ; but 

 where the leaves were linear this number was considerably 

 increased. In the Coal-measure species, Sphenophyllum 

 plurifoliatum, the leaves in each verticil seem to have 

 been eighteen, at least, on the larger stems (Fig. 37). 

 The nodes from which the leaves spring are somewhat 

 enlarged, and the vascular strands pass out almost 

 horizontally (Fig. 36, A). As regards the course of 

 the leaf-trace bundles, our information is chiefly due to 

 M. Renault, who succeeded in working out this question 

 in some of the French forms. In these species, as we 

 have seen above, there are two distinct groups of spiral 

 elements at each angle of the stele — six such groups 

 in all. At the node, one bundle starts from each of 

 these groups, dividing, as it passes through the cortex, 

 into two, three, or four branches, which supplied the 



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