V] 



VENATION 



95 



Dipteris Lohbiana have the meshes imperfectly coupled. But usually where 

 reticulation exists in the adult it is established in the earliest leaves of 

 the individual, as in Dipteris conjugata, Cheiropleuria, and Platy cerium 

 (Fig. 91). So far as it goes the ontogenetic progression indicates that an 

 open venation is primitive, and the closed a derivative state. 



More cogent evidence comes from the stratigraphical sequence of fossils. 

 Reticulation is unknown in Devonian plants. Potonie notes that Stur does 

 not record a single case of reticulate venation from the Flora of the Culm. 

 Meshwork is first seen in the Middle Coal Period, though many of the 



Fig. 92. Clathropteris egypliaca. (Nat. size): a — b, pieces of main ribs 

 in grooves. To the right a small piece of lamina showing the venation. 

 (After Seward.) 



examples of it that have been quoted are now ranked as Pteridosperms: 

 for instance Lotichopteris and Linopteris. But here the vein-fusions are few, 

 and the meshes relatively large. It is not till the Mesozoic Period that 

 reticulation of a higher order, that is with a smaller network within the larger 

 meshes, became prevalent. This state is well illustrated by ClatJiropteris 

 egyptiaca Seward, from the Nubian Sandstone, which has a venation that 

 may be matched by many modern Ferns (Fig. 92). From these facts it seems 

 necessarily to follow that open venation was primitive, and that reticulation 

 %vith progressively smaller meshes zvas derivative. The physiological advantage 



