500 FILICALES 



xylem band, flattened on its inner and outer faces. These strands branch 

 off from the central stele with the minimum of disturbance, after the 

 manner of the strobiloid Pteridophytes. A similarly simple origin of the 

 leaf-trace has been seen also in species of Botryopteris, in which the stele 

 is as little differentiated as in Grammatopteris : there is indeed an entire 

 absence of well-marked protoxylem in the stele of Botryopteris. In 

 Zygopteris also the origin of the leaf-trace is essentially the same, though 

 here the matter is complicated by the curious differentiation of the 

 xylem of the stele : there is an outer band consisting of larger, scalariform 



DC I 



see 



FIG. 270. 



Zygopteris Grayi. Transverse section of stele, showing wood and remains of phloem. 

 1-5 the' five angles of the wood, from which leaf- traces are given off, in order of the 

 phyllotaxis, No. 5 belonging to the lowest of the series, x, principal ring of xylem ; 

 jrz, small tracheides of internal xylem ; j;e, small trachetdes at periphery ; //;, phloem ; 

 r, base of adventitious root. X 14. Will. Coll., 1919, B. (From Scott's Studies in Fossil 

 Botany.) 



tracheides, and a central core consisting of parenchyma together with a 

 system of smaller tracheides : both of these contribute to the strand of 

 the leaf-trace, which is abstricted off from the ray-like projections of the 

 cauline stele (Fig. 270). A new genus from the lower Coal Measures 

 has recently been described by Scott, 1 which is characterised by radially j 

 seriated wood, apparently of a secondary character : in other respects it 

 had much in common with Zygopteris. This is the first evidence of 

 secondary thickening in the Botryopterideae : the fossil has been named 

 Botrychioxylon \ but as the sporophylls have not yet been described, 

 this very allusive name must be understood only to convey the fact that i 

 it is a Botryopterid showing secondary growth, just as Botrychium is an 



l jonrn. fi. Micr. Soc., 1906, p. 519. 



