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XXI. — On Leaf- Architecture as illuminated by a Study of Pteridophyta. By 

 F. O. Bower, D.Sc, F.R.S., Regius Professor of Botany in the University of 

 Glasgow. (With One Plate.) 



(MS. received March 6, 1916. Read March 6, 1916. Issued separately November 8, 1916.) 



The expression "architecture" as applied to the leaf was introduced by Prantl 

 in his monograph on the Hymenophyllacese.* It may be adopted as connoting 

 the sum of the facts of construction of leaves ; together with those principles 

 or methods deduced from them, upon which we find the leaf to be built up. 

 The varieties of size, form, and complexity of leaves appear infinite ; but simi- 

 larities in the scheme of their construction are obvious. It cannot be assumed 

 that where similarities occur they are necessarily due to immediate community of 

 descent. They may or may not be. Parallel development under similar conditions 

 may be, and probably has been often, the source of such similarity. But even so 

 it may be possible to connect the simpler and the more complex within the several 

 lines of nearer relationship, and a study of several such lines may be expected to 

 disclose certain underlying principles or methods which have ruled in the construc- 

 tion of foliar organs at large. The recognition of these, in their evolutionary aspect, 

 is the proper basis for a scientific knowledge of leaf-architecture. 



There are three chief avenues which may lead to such knowledge : — 



(1) A comparative study may be made of adult leaves, in the mature state or in 

 the course of their individual development, in a large number of different types. 



(2) A study may be made of the juvenile leaves of the individual, and the 

 gradual steps be traced up to the adult form. Comparison may then be made of the 

 results in various forms related or systematically apart, and especially in the case 

 of those types that are believed to be relatively primitive. 



(3) The results of such comparisons may be placed in relation to the fossil 

 record, and any conclusions thus obtained as to phyletic progression may be checked 

 accordingly. 



All of these avenues should be pursued in order to arrive at a scientific knowledge 

 of the " architecture " of leaves in Vascular Plants. Naturally the first of these took 

 priority in the history of the science ; and unfortunately it was first practised for 

 the most part upon the Higher Flowering Plants. This often led to the interpretation 

 of the lower in terms of the higher, and to the consequent failure to recognise the 

 full significance of the facts observed. In some measure this has been corrected by 

 the pursuit of the third avenue of study, viz. by the comparison of the fossils. 

 The flattened form of the leaf-blade, which has led to its very perfect preservation 



* Leipzig, 1875, p. 7. 

 TRANS. ROY. SOC. EDIN., VOL. LI, PART III (NO. 21V 97 



