The Fossil Plants of the Coal Measures. 7$ 



has kindly given me the following outline of his views on 

 the structure of his R. Williamsoni : — 



"The structure of the petiole of Rachiopieris Williamsoni 

 " resembles in many respects that of Myeloxylon ; but the vascular 

 " bundles show certain well-marked peculiarities, and a divergence 

 " from those of Brongniart's genus Myeloxylon, which seems to 

 " justify a specific separation. Myeloxylon agrees with Cycads 

 "rather than with Ferns. Racheopteris Williamsoni approaches 

 " much more closely to the typical fern-bundle, and is, therefore, 

 " regarded as a fern petiole. 



"The two plants agree in (1) the nature of the hypodermal 

 " tissue, consisting of alternating bands of sclerenchyma and paren- 

 chyma; (2) in the possession of larger gum (?) canals in the 

 " ground tissue. Their most important differences may be briefly 

 " stated as follows : — 



u In Myeloxylon the bundles of vascular tissue are collateral, 

 "and the protoxylem is placed next to the phloem ; in Rachiopieris 

 " Williamsoni the bundles are concentric, and they agree in 

 "position with that of the ferns. In Myeloxylon there are no 

 "parenchymatous elements associated with the xylem vessels. In 

 " R. Williamsoni there is much xylem parenchyma; another 

 " marked difference consists in the occurrence of regularly disposed 

 " canals surrounding the xylem in R. Williamsoni. These do not 

 " occur in Myeloxylon. The specimens which have been examined 

 "of the new species show these canals in various stages of 

 " development. They are quite distinct from the larger canals 

 " scattered in the ground tissue, and are regularly arranged 

 " towards the periphery of the phloem in each bundle." 



Some of the differences here recorded are easily seen. 

 Others are not so clear in my specimens. Of course the 

 most conspicuous one is the existence of collateral bundles 

 in one case, and of circumferential ones in the other. But 

 e ven here we must remember that we have collateral 

 bundles in ferns {e.g., Osmunda), and De Bary has found 

 circumferential ones in a Cycad. Hence, the question arises, 

 did these differences always possess the same distinctive 

 value that they may do now ? Schenck and Solms-Laubach 



