204 CLADOXYLEAE [CH. 



ii. Leaves. Before describing a second type of stem referred 

 to Cladoxylon it is important to consider briefly sucb evidence 

 as we have as to the vascular supply of the leaves. Nothing is 

 known of the reproductive organs and there is no satisfactory 

 information with regard to the form of the fronds. Solms-Laubach 

 has described the only known example of a lateral branch of a 

 Cladoxylon stem (fig. 459, E) : this has a single concentric vascular 

 strand of plate-hke form with two blunt projections and there 

 are four protoxylem-groups, two in the angle of the plate and 

 two at the base of the projections. The structure is essentially 

 fern-hke; the xylem is wholly primary. This type of vascular 

 strand agrees fairly closely with that of a petiole described by 

 Unger as Megalorhachis elliptica, a section of which is in the Museum 

 of the Geological Survey^. The petiole is oval in section and 

 laterally winged, and the meristele is tangentially elongated and 

 has two blunt projections almost identical with those in fig. 459, F. 

 There is no evidence as to the nature of the supporting stem, 

 but there can be httle doubt as to the close connexion with 

 Megalorhachis and the section shown in fig. 459, F. In a note 

 published in 1908 P. Bertrand stated that he had identified 

 several of Unger's genera as stems which bore leaf-traces having 

 the form and structure of Clepsydropsis, one of the types referred 

 by Unger to the Rhachiopterideae and described in the second 

 volume of this work^ as a Coenopteridean petiole. Bertrand 

 points out that in the oval or plate-hke steles of Cladoxylon, 

 Arctof odium, Hierogramma, etc., there is a single protoxylem 

 group near the distal end of the primary xylem, and he adds 

 that the leaf-traces were formed of strands cut off from the distal 

 portions of the vascular plates. Similarly the hour-glass-hke 

 leaf-trace in the primary rachis of Clefsydrofsis gives off from 

 each end a ring of xylem to supply a secondary rachis. These 

 laterally detached annular strands are, he believes, similar to 

 the leaf -trace cut off from the steles in a Cladoxylon stem. The 

 conclusion is that Cladoxylon is a fern stem and its leaf-trace 

 represents the simplest form of the Clefsydropsis type, namely 

 an oval bundle of xylem with a central protoxylem, which is 



1 No. 15870. Unger and Kichter (56) B. PI. vn. figs. 19—21. 



2 Page 472, fig. 324. 



