V] 



CONCLUSION 



103 



primitive types. But in those where a widening leaf-expanse has been formed 

 occasional lateral fusions of the veins led to a connected network of coarser 

 meshes. This provides for a more equal distribution of water over the en- 

 larged area. Later types show in their smaller-meshed reticulum a still more 

 efficient conducting system within the expanded lamina. Such reticulation 

 usually appears in those specially widened leaf-expanses characteristic of life 

 under forest shade: as in Hypoderris, and Christenseiiia. This type of vena- 

 tion first became prevalent in the Mesozoic Period, thus coinciding with the 

 earliest records of broad-leaved trees. It is significant that these afford a 

 more complete forest-canopy than the narrow-leaved vegetation of earlier 



Fig. 98. Asterocalamites 

 scrobkulatus, Schlo- 

 theim,from the Culm. 

 Fragment of a leafy 

 shoot, reduced to half 

 its natural size. (After 

 Stur, from Zeiller.) 



Fig. 99. Types of leaf in the Sphenophylleae. ^ = a 

 leaf-whorl of Sphenophyllnm ciineifolitim, and one 

 leaf of it somewhat enlarged. B = 3. leaf- whorl of 

 Sphenophyllwn teiierrinmin. C^Sphenophyllum 

 verticillatum (from Potonie's Pflanzen-Palaeonto- 

 logie). D—T7-izygia speciosa, Royle, from the 

 Glossopteris-iz.c\es of India. (After O. Feistmantel.) 



epochs. Notwithstanding these advances, Modern Ferns show a peculiar 

 conservatism of structure, retaining in many cases the features held as primi- 

 tive in the sequence thus recognised. But on the other hand they show in 

 their most advanced types such form and structure as can only be matched 

 among the Higher Flowering Plants. Nevertheless that advanced structure 

 can be referred, through the steps of reasonable comparison as above indi- 

 cated, and with biological probability, to an origin from a simple, single- 

 veined leaf. But this type was amplified by distal dichotomy, by webbing 

 of the resulting lobes laterally, and by fusion of the veins to form a reticulum, 

 associated as these features are with continued apical growth. It is this 

 prevalent conservatism, combined with the various factors of advance, which 



