CHAPTER XXVIII. 



II. SPHENOPHYLLALES. 



THIS second phylum of the sporangiophoric Pteridophytes includes the 

 title-family of extinct fossil plants, the Sphenophylleae, and associated with 

 them, though perhaps somewhat aloof, as differing in certain important 

 features, is the living family of the Psilotaceae : this contains the genera 

 Psilotum and Tmesipteris. Certain other imperfectly known fossils may 

 also find their best place in this relationship. The Sphenophyllales are 

 characterised by having a dominant axis, with protostelic structure, which 

 bears leaves of moderate size, with more or less furcate branching, and 

 arranged either in whorls (Sphenophylluni) or alternate (Psilotaceae). An 

 important distinctive character is the insertion of the sporangiophores not 

 directly on the axis, but upon the appendages : they are thus marked 

 off clearly from the Equisetales, notwithstanding that they have many 

 points of resemblance to them : these points are more marked in the 

 Sphenophylleae, while the relation of the Psilotaceae is rather towards 

 the Lycopodiales. The whole phylum thus occupies an intermediate, or 

 perhaps a central position, which gives its study a very special interest. 



A. SPHENOPHYLLEAE. 



This ancient and long extinct family is represented according to present 

 knowledge by the undivided genus Sphenophyllum : but associated more 

 or less distinctly with it is the complex strobilus known as Ckeirostrobus. 

 The Sphenophylleae as at present known dated from the Calciferous 

 Sandstone series of the Lower Carboniferous formation, and extended 

 upwards to the Permian. They were plants of straggling habit, with the 

 usual vegetative region preceding the spore-producing parts : these were 

 commonly borne upon definite terminal strobili, but at least one case is 

 known where the definition of the vegetative and reproductive regions was 

 less clearly marked. In the case of Cheirostrobus the vegetative region 

 is still unknown 



