LEAF-ARCHITECTURE AS ILLUMINATED BY A STUDY OF PTERIDOPHYTA. 705 



appear that the study of the juvenile leaves in relatively primitive Vascular Plants 

 is not the least valuable of them in eliciting the phyletic story of leaf-architecture. 

 The view has been adopted throughout that the juvenile leaves of the Pteridophyta 

 are rudimentary, not arrested. For they are free-living leaves, that have never 

 been controlled, like the cotyledons of Seed-Plants, by the exigencies of packing 

 within a seed, or by adaptation to storage. It is held that the adult leaf is the result 

 of promoted development from this relatively simple source. The theoretical position 

 is entertained that the progression which successive juvenile leaves show with 

 greater or less perfection, from equal dichotomy to sympodial and ultimately to 

 monopodial branching, reflects with some degree of accuracy a probable story of 

 phyletic advance in leaf-architecture. The microphyllous Pteridophytes would then, 

 speaking generally, be types which had progressed less far, though in some cases 

 simple leaves appear to have become so by reduction. This may have been so in 

 Equisetum, and it is believed to be the case for the simple leaves of some Ferns and 

 Flowering Plants. But such an interpretation will not apply to the Lycopodiales : 

 for in them the leaf is always simple, and the earliest fossils of the family bear this 

 out. They appear to have retained a primitive simplicity of the leaf. Excepting , 

 then, such cases as are probably reduced, a general working hypothesis may be 

 adopted, that the progressive steps seen with varying completeness in the successive 

 juvenile leaves of the individual, and reflected by comparison of the adult leaves in 

 various early vascular types, indicate the steps of elaboration which prevailed in 

 the evolution of foliar structures in the Sporophyte. 



The complex scheme thus set up in the megaphyllous types was suitable enough 

 for hygrophytic vegetation, such as the Filicales and Pteridosperms. But it was 

 liable to modification under exposed subaerial conditions. The phyletic history of 

 the Angiosperms may be uncertain, but the probability is that they sprang from a 

 megaphyllous rather than a microphyllous source. And already among megaphyllous 

 types we have seen evidences of certain modifications which would have the effect 

 of producing a leaf like that of the Angiosperms. The chief of these are apical 

 arrest, with omission of the phyletically oldest but biologically most vulnerable 

 distal region, and the substitution of intercalary for apical growth. Thus, we may 

 believe, there was produced a type of leaf with various added advantages ; such as 

 an early maturation of the apex, a more robust construction, a capacity for close 

 packing in the bud, and a more adaptable development of form to meet the various 

 biological needs of subaerial life. In fact, the biological probability that such changes 

 should supervene as amendments of the Filical construction lends support to the 

 conclusions arrived at on a basis of comparison. The application of the results 

 obtained from the Pteridophyta to the leaves of Flowering Plants can here be only 

 suggested in general terms. It is hoped on some future occasion to develop the 

 points of comparison on the basis of fresh observations on them. But if the com- 

 parisons here instituted are correct, they indicate as a general statement for Vascular 



TRANS. ROY. SOC. EDIN., VOL. LI, PART III (NO. 21). 103 



