LEAF- ARCHITECTURE AS ILLUMINATED BY A STUDY OF PTERIDOPHYTA. 689 



(Plate-fig. C). The same again, with the added point 'of a spiral curvature of the 

 shanks, is seen in Camptopteris spiralis (Plate-fig. D). All of these are examples 

 of sympodial dichotomies, but showing in increasing degree the consequences of the 

 anadromic helicoid development. 



Biologically these forms are interesting as showing how from a flattened organ 

 the advantages of a radial symmetry may be acquired. The modern forms are 

 rhizomatous, with long-stalked upright leaves. Probably the fossils were the same. 

 The problem then would be how best to expose the assimilating surface. In Matonia 

 the helicoid sympodium is expanded almost into the form of a widely open funnel, 

 the effect of which is lost in pressed specimens.* This is still more the effect of the 



a h 



Fig. 22.— a, juvenile leaf of Cheiropleuria ; b, a similar leaf of Platycerium Vcitchii. ( x 3. 



webbed expanse of Dipteris conjugata.] But in Camptopteris, by the adoption of 

 the spiral curvature of the sympodia, the pinnae radiate outwards like bristles 

 of a spiral wire bottle-brush. A good substitute for a radial vegetative shoot 

 is the result. 



On the other hand, it is only a step in simplification from the condition of the 

 leaf of D. conjugata to that in the allied Cheiropleuria. \ There the adult blade 

 may be entire, or variously lobed. But its base shows the dichotomy very plainly, 

 while it appears in the juvenile leaf in almost exactly the same way as in Dipteris 

 (fig. 22, a). It may further be noted that Platycerium, in which the adult fertile 

 leaves show often very equal dichotomy, has in its juvenile leaves a venation closely 

 similar to that of Cheiropleuria (fig. 22, b). 



* Compare Seward, Fossil Plants, vol. ii, p. 293, fig. 228. 

 X Ann. ofBot., 1915, p. 495. 

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



f See Christ, Geographie der Fame, fig. 5. 



101 



