LEAF-ARCHITECTURE AS ILLUMINATED BY A STUDY OF PTERIDOPHYTA. 667 



relation to the venation, and to the developments seen at the leaf-base ; for all of 

 these points have their bearing, not only on this individual case, but also upon the 

 behaviour of leaves at large. 



The origin of the lower pinnae of the mature leaf is known to be monopodia!* 

 They arise in acropetal sequence below the apex, and in the already massive organ 

 from which they spring they cannot be traced as having any definite relation to its 

 apical segmentation. But passing up from the base of the whole leaf to the distal 

 end there is a gradual transition, similar to that already known in other Fern leaves ; 

 this leads to a mode of origin of the last branchings being referable to a sympodial 

 dichotomy, of the same type as that seen in the juvenile leaves. The base of the 

 terminal lobe often gives indication of the relation which exists between these later 



Fig. 2. — Portions of adult leaves of Osmunda rcgalis, showing the relation of 

 pinnules to the venation and to the leaf outline, (x 4.) 



branchings of the leaf and its venation. The branch-system resulting from the 

 dichotomy of each single lateral vein may in the distal part of the lobe be included 

 within the margin of that lobe ; but towards its base a convex crenation may some- 

 times be seen, which includes the whole of such a branch-system (fig. 2, a). Lower 

 down a more definitely projecting lobe may be found, and lower lobes still may 

 successively be elongated outwards and their venation be sympodially developed. 

 The separation of such lobes from the terminal lobe may be completed by involution 

 of the margin down to the midrib. Such steps, which link together the branchings 

 of the venation with the origin of the separate pinnae, are seen at the base of the 

 leaflet shown in fig. 2, b. Such a relation is found to hold very widely in Ferns. It 

 shows that even a pinnate leaf is constructed on the basis of sympodial dichotomy, 

 while the pinnae thus produced may be alternate, or apparently opposite, and owe 

 their origin to monopodial branching. 



This seemingly alternate or paired disposal of the pinnae is prefigured by similar 

 dispositions of the lateral veins. Uniformity in the mutual relations of the veins is 

 not necessarily found throughout a single pinna. This is illustrated by fig. 3. In 



* Phil. Trans., part ii, 1884, p. 570. 



