i22 STUDIES IN FOSSIL BOTANY 



Lepidodendron, as we shall see in the next chapter. 

 While thus combining certain characters of the Horse- 

 tails and the Club- mosses, Cheirostrobus shows the 

 most marked affinities with the Sphenophylleae in the 

 following characters : the arrangement of the append- 

 ages in superposed verticils, the palmatifid segmenta- 

 tion of the leaves (sporophylls), the repeated branching 

 of the leaf-traces within the cortex, and the relation of 

 the sporangiophores to the sterile segments or bracts. 

 In Cheirostrobus it is evident that the sporangiophores 

 are ventral or superior segments of the same leaf of 

 which the sterile bracts are the dorsal or inferior 

 segments. The same relation holds good for Spheno- 

 phyllum Dawsoni, and still more clearly for Bowmanites 

 Rb'meri, where the homology between sporangiophores 

 and bracts is patent. In fact, this latter species 

 occupies, in this respect, exactly an intermediate 

 position between Sphenophyllum Dawsoni and Cheiro- 

 strobus. Sphenophyllum fertile, in which both dorsal 

 and ventral segments are fertile, resembles Cheirostrobus 

 in their palmatifid branching. The course of the 

 vascular bundles supplying the sporangiophores and 

 bracts is essentially the same in Sphenophyllum and 

 Cheirostrobus, though necessarily more complex in 

 the latter. 



The general conclusion, to which the various 

 characters point, is that Cheirostrobus has more in 

 common with Sphenophyllum than with any other group 

 of plants at present known, and that, though the former 

 genus must constitute the type of a distinct family, it 

 is most naturally placed in the same main division of 

 Cryptogams, which we may call the Sphenophyllales. 



