LEAF-ARCHITECTURE AS ILLUMINATED BY A STUDY OF PTERIDOPHYTA. 661 



While the statements of Goebel in his Organography present the purely 

 morphological position, Potonie had been approaching questions of leaf-architecture 

 from the avenue of palaeontological comparison.* He regarded dichotomy as the 

 primitive type of branching, and monopodial (pinnate) as the derivative. He notes 

 the greater prevalence of the former in Palaeozoic than in more modern times, and 

 that it still exists, though less frequently, in the Higher Plants, being represented 

 atavistically in their cotyledons. He discusses why the pinnate should have replaced 

 the dichotomous, and sees the explanation in the point that a dichotomous blade 

 would result in a circular surface of leaf-expanse, while a pinnate blade approaches 

 the oval, which has mechanical-physiological superiority, and is better fitted to 

 resist leverages of weight and wind-pressure. He passes from Ferns to Vascular 

 Plants generally, and asserts (provisionally) that all monopodial branchings origi- 

 nated phylogenetically from dichotomies. This question will come up for discussion 

 towards the end of this memoir. 



Turning more especially to the leaves of Ferns, Potonie t discusses their archi- 

 tecture in the ancient fossils, as regards both venation and branching. He notes as 

 some of the chief features in early Ferns : (l) the preponderance of dichotomy in 

 them ; (2) their frequently unsymmetrical development by interchange of forking 

 and pinnation, and transitions from one to the other ; (3) the frequent occurrence of 

 lobes of the ultimate order between those of the penultimate order ; and (4) the 

 frequently large size of the katadromic pinnule. In addition to these features he 

 remarks also that most Palaeozoic fronds have katadromic construction, while in 

 recent types the anadromic greatly preponderates. Also that in the earliest a 

 midrib is absent in the ultimate pinnules, while in later forms it is present. Further, 

 that reticulation becomes more prevalent from lower to higher horizons. He notes 

 that Stur in his Culm-Flora does not represent a single case of reticulation : this is 

 initiated in the Middle Coal Measures, and becomes more prevalent upwards, while 

 the highest type of reticulation, with smaller meshes within larger areolae, occurs 

 first in the Mesozoic period. 



Such statements as these, based upon a wide knowledge of fossils, supply a 

 foundation upon which to work. They serve as checks to comparative conclusions ; 

 and it is a very reassuring fact that the results already attained by comparison of 

 living forms are in substantial accord with them. A real advance has thus been 

 made along the two avenues of study on the adult leaves and on the fossil types. 

 But a knowledge of the actual facts as to the structure of the primordial leaves is 

 still very incomplete. It may be a question how far their characters will bear an 

 atavistic interpretation. But on this it is impossible to approach any opinion until 

 the facts are before us. These will now be described so far as they are available. 



* " Die Beziehungen zwischen dem echt-gabeligen und dem fiederigen Wedel- Auf bau der Fame," Ber. d. d. Bot., 

 Ges. xiii, 1895, p. 244 ; also Lehrbuch der Pflanzen-Palaeontologie, Berlin, 1899, p. 110, etc. 

 t Lehrbuch, p. 110, etc. 



