V] 



VENATION 



93 



follows from the fact that the character is not constant even in the same individual in 

 Trichomanes (Mettenius(67), pp. 415, 416). A like inconstancy is seen in the juvenile leaves 

 of Gleichetiia (Bower(77), p. 684, Fig. 18). It may be illustrated in many other examples, 

 notably in the genus Aspidiiim {Nephrodium, Dryopteris). At best this character can only 

 be of restricted use. C. Christensen states (83, p. 5, 1920) that "a stable arrangement of 

 the ribs can only be found in leaves that are divided to a certain degree, at least twice. 

 The sequence of the primary pinnae is unavailable." Whatever its value may be in giving 

 minute systematic distinctions, it cannot be used generally. 



Venation 

 Characters of comparative importance are found in the vein-endings, and 

 the relations of the veins throughout their course. IXtJtiem;igin^f the expanded 

 bla_da±)e as already suggested,^nd if all broad leaf-areas of Ferns have re- 

 sulted ultimately from webbing of narrow dichotomously branched lobes, tbgJT 

 the primitive venation should have free endings to all the veins. This is 

 sometimes called an open venation, as distinct from a closed venation, where 

 the veins may be fused into loops. Any such vein-fusions should be held as 

 derivative, and secondary. Comparison of the leaves of those Ferns which 

 on other grounds are held to be primitive shows that in them the veins end 

 free. For instance, Botryckium, Helminthostachys, Marattia, Angiopteris, 

 Sckizaea, Gleichenia, and almost all of the Cyatheaceae and Hymeno- 

 phyllaceae, have an open venation, with free vein-endings, and no fusions. 

 A first step towards a closed venation is the formation of distal loops. This 

 may result in a continuous intramarginal commissure, such as that seen in 

 Marsilia (Fig. 89). Vein-fusions may, however, be initiated in other ways, 



Fig. 89. Successive types of juvenile 

 leaves oi Marsilia. (After Brann.) 



Fig. 90. Abnormal leaf of Marsilia qtiadri- 

 folia, with .six jnnnae. (After Velenovsky.) 



