V] 



WEBBING 



contains only one vein, as in the Sphenopteris type. It appears that later the 

 segments became progressively 

 webbed by lateral fusion. More- 

 over the margin often shows 

 distal teeth, each containing a 

 vein-ending, which marks the 

 constituent segment (Fig. jyX 

 If then progressive webbing had 

 really taken place the vena- 

 tion as actually observed would 

 be a more direct guide to the 

 branching which underlies the 

 structure of the leaf, lobe, or 

 segment, than is the mere out- 

 line. In deeply cut leaves the 

 two may coincide, but in fully 

 webbed leaves the margin may 

 be quite entire, though with 

 highly branched venation, as in the Hart's Tongue, or in Trichomanes 

 (Fig. 78). In that case venation and not the outline will be the safe guide 

 in a morphological analysis of the blade whether of juvenile or of adult 

 I 2 .1 4 5 6 7 



Fig. 76. Juvenile leaf of Todea superba, showing 

 single-nerved segments all separate. ( x 3.) 



10 98 



Fig. 77. Successive stages of development of the juvenile leaves of Anemia adiantifolia. ( x 4.) 



leaves. This line of comparison can be followed even in the fossils, for in 

 them the course of the veins is often well preserved. The general conclusion 

 following from such considerations is : that in a primitive type of leaf- 

 construction the segments resulting from distal branching were separate 

 laterally one from another: but that by progressive webbing they became 

 coherent laterally to form broader expanses in later and derivative types. 



