THE FERNS 145 



sisting merely of a solid column of uniform wood, 

 surrounded by a layer of bast; in others, the cen- 

 tral part of the wood was different from the outer 

 zone, as in the Permian Osniundacese; and in 

 others again, the centre was occupied by a mix- 

 ture of wood and pith, as in the Wealden Osmund- 

 ites mentioned above. In these more complex 

 cases the woody column was apt to lose its simple 

 cylindrical form and to become fluted and grooved, 

 often to an extreme extent. Sometimes the 

 branching of the stem was a mere bifurcation, as 

 it is in the modern Bracken, but in other cases 

 the branches were axillary, springing from the 

 angle between stem and leaf, as happens in Filmy 

 Ferns at the present day, though otherwise not a 

 Fern-character. The petioles were in many cases 

 thicker than the stem that bore them, and the 

 single vascular strand of the petiole often had a 

 complicated structure. A frequent peculiarity is 

 that two rows of leaflets were given off on each 

 side of the main leaf -stalk; this strange arrange- 

 ment, however, may only have belonged to the 

 fertile fronds, for it was commonly the case in 

 this group that the sporangia were borne on special 

 fertile fronds without leaflets; in this respect 

 these early Ferns were already rather advanced, 

 compared with most of their present-day relations. 

 In other cases, however, the fructification was 

 on the under side of ordinary leaves. 



