84 



LEAF-ARCHITECTURE OF FERNS 



[CH. 



entire, as in S. verticillatuin. Potonie states(72, p. 176) that on passing from 

 lower to later geological horizons the size of the leaves increases, but the 

 degree of their cutting decreases. The ancient 5. tenerrinmui has narrow linear 



Fig. 75. Yi2\Alo{ \&^ve.'A o{ Hymenophyllum. B= H. dilaialuni,S\w. C= /7. atts/ra/e, W\Ud. In B 

 and C the single-veined segments are separate one from another, which is held to be a more 

 primitive condition. (After Sadebeck, from Engler and Prantl.) 



radiating segments : the more modern S. Thoni has large undivided leaves 

 (Fig. 99, p. 103). These facts are important for comparison with the Ferns. 

 It seems probable that a primitive condition of the leaves in Ferns as well as 

 in Sphenophylls was that in which each segment is separate laterally, and 



