694 



PROFESSOR F. O. BOWER ON 



dichotomy — be repeated on the same plan (compare/, g), the result is reached as in 

 (It) or (i). From these to the adult leaf of Pteridium the course is plain enough. 

 It is interesting to note the close similarity of the juvenile leaves of Dennstsedtia 

 punctilobula, as shown by Conard.* 



In this description of juvenile leaves of Ferns the types with reticulate venation 

 have been purposely omitted, as they are with reason believed to be derivative 

 forms in respect of that venation. But they must not be wholly neglected. Already 

 the juvenile leaves of Dipteris have been described, and there the venation was 

 found to be fundamentally and equally dichotomous (D. conjugate), or a sympodial 

 dichotomy (D. Lobbiana), with reticulate loops superposed upon that branching. 



/ 



Fig. 26 bis. — Juvenile leaves of Ceratopteris thalictroides. ( x 4.) 



Another example has been examined in Ceratopteris, in which the seedling leaves 

 had already been figured by KNY.t The youngest leaves have a simple, unbranched, 

 median strand (fig. 26 bis, a) ; slightly more advanced leaves may show dichotomy, 

 with free branches (b), but later leaves may have the distal ends connected by a 

 loop (c), while free-ending branchlets radiate outwards from it (c, d). From this 

 state further advances may be made connecting these branchlets into further loops, 

 and also by commissures between the main veins (e,f). But still the fundamental 

 dichotomy may be recognised at the base of the blade, even in relatively advanced 

 leaves, showing that the reticulation is a superstructure upon it. 



It seems unnecessary to repeat in additional examples progressions from the 

 juvenile to the adult leaf which follow substantially similar lines. The above will 

 suffice to illustrate, first for the Leptosporangiate Ferns with open venation, and also 

 for those showing reticulation, how an apparently pinnate leaf may originate from a 

 dichotomous source. A few notes are, however, appended here upon certain out- 



* Carnegie Inst., No. 94, pis. xxiv, xxv. t Parkeriaceen, pi. xxii, figs. 4-8. 



