Sphenophylliim as a branch of Asterophyllites. 157 



considers that the absence of intercellular spaces in the stem 

 of Sphenophylliim negatives the usual explanation that the 

 finely-dissected leaf-form is the result of modification 

 brought about by submersion. Whatever be the cause of 

 such difference in the form of the leaves, may we not regard 

 the Sphenophylloid leaves as having been formed by a 

 coalescence of groups of acicular Asterophyllitean leaves ? 

 Germar's figure {Fig. 3) affords us a good example of a 

 case where the passage from cuneiform to linear leaves may 

 be traced. As Prof. Williamson x has remarked, this expla- 

 nation of the formation of the broad leaves by coalescence 

 of linear ones accounts for the smaller number of the former 

 in each verticil compared to the larger number of the latter. 



The evidence for the close connection between stems 

 having these two types of leaves does not rest merely on 

 specimens which show no internal structure. In Memoir V. 

 of the Phil. Trans. Prof. Williamson has described a series 

 of specimens shewing different stages in the growth of an 

 Asterophyllitean stem ; the stems from which the sections 

 were taken were found with Asterophyllitean leaves attached 

 to their nodes. 



In Plate III. 2 {Figs. 14 — 17) sections are figured where 

 we have a verticil of acicular leaves, sections of which shew 

 a single vascular bundle in each case : in Memoir IX. 3 there 

 is another section described and figured, where we have not 

 only traces of leaves, but in the cortex of the stem are a 

 number of vacant spaces, which were no doubt originally 

 occupied by foliar bundles. Prof. Williamson points out 

 the resemblance between the structures described by himself 

 in these Asterophyllitean stems and those described by 

 Renault in Sphenophylliim stems : in the one case the stems 

 have undoubted Asterophyllitean leaves, and in the other 



1 Phil. Trans. Royal Society. 



1874. 



Pt. V., p. 77. 



2 Phil. Trans. R. S. 1874. 







3 Phil. Trans. R. S. Pt. II. 



1878. 



Plate xxi., Fig. 32. 



