708 LEAF-ARCHITECTURE AS ILLUMINATED BY A STUDY OF PTERIDOPHYTA. 



The obvious rejoinder to this theory, supported as it now is by the study of 

 relatively early fossils, is that it starts from forms of extraordinary complexity. 

 I do not anticipate that any settled conclusion can be arrived at for the present 

 as to the ultimate origin of foliar structures in the Vascular Plants. Perhaps the 

 most valid suggestions may come from newly discovered fossil-types. But in my view 

 the methods pursued in this memoir, based as they are upon the general study 

 of well-known living plants, are preferable meanwhile to comparisons of the mature 

 anatomy of adult organs, in plants which are imperfectly known and appear to have 

 been highly elaborated and specialised. A comparison based upon embryology ; upon 

 the successive juvenile leaves of the individual, leading up to the adult; and upon 

 the form and venation of the adult leaves of all the leading types of Vascular Plants, 

 and checked by reference to the fossil record, offers a footing, however precarious, 

 upon which to approach some reasonably probable conclusion. That provisional prob- 

 ability is, that leaves were initiated as lateral appendages of an axis ; and that the 

 more complex leaves resulted from elaboration of a simpler type. 



DESCRIPTION OF PLATE. 



Fig. A. Leaf of Matonia pectinuta, R. Br. ; reduced to ^ natural size. 

 Fig. B. Leaf of Dipteris conjugata (Kaulf ), Reinw. ; reduced to \ natural size. 



Fig. C. Leaf of Dietyophyllum exile ; much reduced. After Nathorst, from Seward's Fossil Plants. 

 Fig. D. Leaf of Camptopteris spiralis ; much reduced. After Nathorst, from Seward's Fossil Plants. 

 Fig. E. Leaf of Pteris semipmnata, reduced. 



Fig. F. Leaf of Odontopteris minor, Brongn. ; rather less than i natural size. After Zeiller, from 

 Seward's Fossil Plants. 



