64 



OPHIOGLOSSACEAE 



[CH. 



form a sort of midrib. Free vein-endings are seen both within the meshes 

 of the network and also at the leaf-margin. Venation of this type originated 

 only in the Mesozoic Period, and it is held to be secondary and derivative. 

 This, together with the condition of the stele and leaf-trace, marks Ophio- 

 glossiLin as a relatively advanced member of the family. 



The origin of the pinna-trace in the simpler t}'pes of Botrydiiuni is 

 by marginal abstriction from the strands of the 

 midrib. But in the more complicated leaves, where 

 those strands are large with strongly incurved 

 adaxial hooks, the origin may be extra-marginal 

 after the type common in large-leaved Ferns 

 (Vol. I, p. 174, Fig. 170). This is seen in B. vir- 

 ginianum (Fig. 360). In the ternate leaf of Hebnin- 

 thostachys the segregation of the supply to each of 

 the three segments may be accompanied by compli- 

 cated fusions and splittings of the numerous strands. 



The vascular supply to the fertile spike originates in a manner similar to 

 that of the pinna. In certain species of Botrychiinn and in Ophioglossuni, it 

 arises by abstriction from the margin of this meristele: a strand may be 

 supplied from each margin, a fact which was the base of Roeper's theor\' of 



Fig- 359- Diagrams illustrating 

 the origin of the strands which 

 supply the fertile spike in 

 Botrychuim ternatum. The 

 xylem is shaded, the phloem 

 left clear. (After Chrysler.) 



Fig. 360 A. Diagrams illustrating the origin of the strands which supply the first pair of 

 pinnae of the sterile segment in Botrychium virginianum. Fig. 8 follows Fig. 7 in the 

 sequence. (After Chrysler.) 



