LEAF-ARCHITECTURE OF FERNS 



[CH. 



seen in HelinmtliostacJiys (Fig. 8 1 , B), or BotrycJiiitvi (Fig. 8 1 , ^ ). Comparison 

 of the juvenile leaves of other Ferns supports the view that equal dichotomy 

 was the prior, and pi'obably the original state in the construction of the leaf, 

 and that some form or another of dicJiopodium of the viai)i veins is a state 

 derivative from it. 



The dichopodium thus established in the juvenile leaves by unequal 

 development of dichotomous branching may be worked out in the leaves 

 of the adult Fern in various ways. The commonest is by the promotion 

 alternately of the right and left shanks of the forking over the other. The 

 result is a straight or slightly zig-zag scorpioid dichopodium, which appears as 

 a continuation of the stipe, forming the midrib or rachis of the whole blade. 

 It apparently continues the petiole, and the two together then constitute 

 the phyllopodium. This is seen in the Male Shield Fern, and in most of the 



Fig. 8i. A = cotyledon oi BolrycJiiiim virginiaimm. ( >' 4-) j5 = a juvenile \Q2i{ oi Hehninthostachys, 

 probably an actual cotyledon. From collection of Dr Lang. ( x 4.) Here the ontogeny starts from 

 a stage which appears relatively late in the series of Osniunda seen in Fig. 80, or Anemia, Fig. 77. 



British species of Ferns(Fig. 2, p. 3). Frequently,however,the promoted shank 

 may not be alternately right and left, but continuously repeated on the same 

 side. This gives the type of helicoid branching. There may be two different 

 types of this : in one the promoted branch is on the anadromic side : — that 

 is the side directed towards the leaf-apex : in the other it may be on the 

 catadromic side: — that is the side directed towards the leaf-base. The former 

 is seen in the pinnae of Pteris semipinnata, each of which is developed as 

 an ascending helicoid dichopodium (Fig. 82, E). It is also illustrated by 

 Dipteris confugata, and Dictyophyllum exile (Fig. 82, B, C). The latter type 

 is seen in the whole leaf of Matonia pectinata, in which the right and left 

 halves are each a descending helicoid dichopodium (Fig. 82, A). The ex- 

 amples quoted have been selected as bringing the character of the branching 

 prominently forward. But similar features often appear in the details of the 



