V] 



WEBBING 



83 



(II) Comparison of the details of the adult leaves in different species 

 and genera. 



(III) Comparison of the related fossils. 

 All these avenues should be pursued in 



order to arrive at a scientific knowledge of 

 the principles on which leaf-architecture 

 has progressed from simple beginnings : 

 and if the reasoning from the facts be cor- 

 rect, the conclusions from them all should 

 substantially coincide. 



Webbing 



We ought to distinguish at the out- 

 set two factors which do not necessarily 

 advance parallel, though they may fre- 

 quently be found to do so. They are, 

 the venation and the cutting of the leaf- 

 blade. In the simplest unbranched cotyle- 

 donary leaves there may be only a single 

 vein: this is seen in Hyvienopliylluni and 

 Trichoinanes. In the branched adult leaves 

 of these Ferns, as also in the cotyledons and 

 adult leaves of other Ferns, each separate 

 segment contains only a single vein 

 (Fig. 75). This, which may be held as 

 a primitive state, is seen in most of the 

 species of Hyinenophyllnm and Tj'icho- 

 nianes, and in Todea siiperba (Fig. 76). 

 But in others of the Hymenophyllaceae 

 and Osmundaceae, and very commonly 

 elsewhere in Ferns, each segment may 

 contain more than one vein. Various Fie 

 degrees of lateral fusion of narrow, one- 

 veined segments to form a broad many- 

 veined lobe may be found, which clearly demonstrate the close relation 

 between the separate-lobed and the " webbed " conditions. An instructive 

 parallel is found in the Sphenophylls, where there is great variety in the 

 webbing of the leaves. All grades of it can be illustrated in the genus 

 Sphenophylltim, from narrow linear dichotomising segments with a single 

 vein each, as in .S. tenerrimiini, to webbed segments with a marginal tooth 

 at the end of each vein, as in 5. cuneifoliiim. The margin may even be 



6-2 



74. Habit of the genus Trichoinanes, 

 A=Tr. rciii forme, Forst. (After Sade- 

 beck, from Engler and Prantl. ) 



