82 



LEAF-ARCHITFXTURE OF FERNS 



[CH. 



^"'S' 73- Diplolabis Romeri, Solms. Diagrammatic 

 drawing showing the ramification of the primary 

 petiole, as it might be seen in a thick transverse 

 section. /'/'= principal plane of symmetry ; MM= 

 median plane ; aph = aphlebia-trace ; pinn = pinna- 

 trace. (From P. Bertrand, after Gordon.) 



degree. The branches are then arranged in two longitudinal rows, one on 

 either side of a central stalk or rachis, which is continuous below into the 

 stipe or petiole, often of considerable length. The whole of this, including 

 petiole and rachis, may be styled 

 the Phyllopodium^. The usual 

 appearance of the foliar organ in 

 Ferns is thus that of a pinnate, 

 petiolate leaf, comparable with 

 that of some leaves of Dicoty- 

 ledons (see Fig. 2, p. 3). But others 

 may be fan-like in construction, 

 with lobes radiating out from a 

 common point, or with veins 

 traversing in similar fashion a 

 webbed, that is a simple, blade or 

 lamina (Fig. 74, A). Many inter- 

 mediate conditions may also exi-st, 

 both in the webbing and the 

 cutting of the leaf, and in the 

 disposal of the veins, though with 

 persistent dorsiventral symmetry. 

 Comparison of these raises the whole question of the architecture of the 

 leaves of the Filicales, and the recognition of the principles which underlie 

 their diverse construction. It will be found that, however complex, they are 

 all deducible from dichotomous branching of a primordial leaf. The fact 

 that equal dichotomy of the leaf is also seen in the Sphenophyllales and 

 Equisetales indicates that this has been a widespread feature among primi- 

 tive Vascular Plants. 



There are three chief avenues for the study of the leaves of Ferns, which 

 may lead to a knowledge of the principles that have ruled in their evolution and 

 resulted in their architecture as now seen in the adult state [Bower(7f-)]. The 

 same method would apply equally for other plants in which elaborate leaf- 

 architecture is .seen, provided the necessary facts were available. They are : — 



(I) Comparison of the juvenile with the adult leaves of the individual 

 plant. 



1 This term was introduced in 1884 (71, p. 565) in the sense here applied in the text. It ranks in con- 

 struction and in meaning with such words as "dichopodium," "sympodium," "monopodium." The 

 name connotes the dominant axis or rachis of a branch-system of foliar character, bearing appendages 

 the relation of which to the rachis or axis (phyllopodium) may be either that of monopodial branching 

 or of sympodial dichotomy. The same word has since been used by Luerssen (xiii, p. 10) as con- 

 noting the base only of the leaf-stalk, which often persists as a scar upon the rhizome. It is not used 

 in that sense here ; and priority as well as preference of meaning and construction are claimed for its 

 use in the sense above defined. 



