408 



SPHENOPHYLLALES. B. PSILOTACEAE 



of Sphenophyllum in which the arrangement of the sporangia is in a group 

 disposed radially around a central attachment a condition comparable 

 with that of the Calamarians, and which was 

 probably a relatively primitive state. Finding these 

 two features combined in the same plant gives to 

 both additional weight. But they are also combined 

 in that other series which, following the suggestion of 

 Thomas, are here included with the Sphenophyllales, 

 viz. the Psilotaceae. These will now be described, 

 and the general discussion of the morphology of the 

 sporangiophoric Pteridophytes will be reserved till it 

 can be illuminated by the facts which these living 

 genera supply. 



FIG. 224. 



B. PSILOTACEAE. 



Diagram cf the vascular 

 supply to the sterile lobes 

 (st), and to the sporangio- 

 phores (f) in Cheirostrobus. 



The genera Tmesipte?-is and Psilotum are the only 

 living representatives of this peculiar and somewhat 

 isolated family, while there is nothing known among 

 Fossils which can with any certainty be ascribed to 



it. They have commonly been classed with the Lycopodiales, and, as 

 we shall see, there are many undoubted points of resemblance in that 



FIG. 225. 



Calamostachys Binneyana. Transverse section of axis of cone, showing stele and part 

 of cortex. Surrounding the pith there are six bundles, in groups of two, with secondary 

 wood. /jr = protoxylem groups. X about 60. Phil. Trans. W. andS. Will. Coll., 1016. 

 (From Scott's Studies in Fossil Botany.') 



direction. But increasing knowledge of the Sphenophylleae, as well as 

 of the Psilotaceae themselves, has indicated a more natural position of 

 both together in the phylum of the Sphenophyllales. The two genera of 

 the Psilotaceae are so similar in their general characters that there is no 

 doubt of their close affinity : on the other hand the differences of detail 



