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PROFESSOR F. O. BOWER ON 



(Plate-fig. F), while it may there be combined with an equal dichotomy of the 

 whole blade. 



Lastly, this peculiarity of sporadic equal dichotomy must be considered. We see 

 it in fossils. It is frequent in living Ferns. It appears sometimes at the distal ends 

 of the pinnules ; sometimes it involves whole pinnse, or the whole distal region of 

 the blade, or even the lower part of the leaf, so that in extreme cases the appearance 

 is as of two whole leaves borne on a single stalk. In one degree or another it is a 

 common abnormality, and provides the " forma fur cata" of Fern collectors. It has 

 already been described above in Osmunda (fig. 4), and it is found in Polypodium, 

 Scolopendrium, Nephr odium, and Athyrium, while it appears also frequently in 

 Nephrolepis. An unexpected but very striking example is seen in Platyzoma, in 

 a furcate form sent by Baron F. von Muller to the Kew Herbarium. It has also 

 been examined by Mr J. M. Thompson,* in specimens supplied from the Sydney 



Fig. 31. — Part of a pinna of Ptcris semipinnata, showing the relation 

 of two of the pinnules to a pinna, and to its venation. ( x 3.) 



Herbarium. It may be there seen to affect the distal end of the leaf only, or to 

 extend lower down to the petiole. It is shown also in the fossils, as in the 

 Pteridosperm Odontopteris (Plate-fig. F). Definite furcate races have been estab- 

 lished, where this character became permanent, as in Nephrolepis. Such a race 

 of Nephr odium molle was cultivated some years ago in the Glasgow Botanic Garden. 

 The petiole in such forked leaves showed a very perfect division of the vascular 

 supply into equal parts. Such forkings .may be interpreted as the result of a 

 reversion, in the 6ase of individual branchings of a sympodium, to the primitive 

 equal dichotomy. Their existence in so many genera and species may be held as an 

 indication that, even where the sympodial development is strongly marked, the 

 branching is still essentially dichotomous, and capable of quite equal development 

 of its shanks. 



The interest in the various leaf-forms analysed in the above pages lies not so 

 much in their mere existence, as in the fact that they are all capable of elucidation 

 in terms of the underlying dichotomy. So that a general statement may be 

 formulated with regard to the construction of the leaf in Ferns, including these 

 peculiarities : — That the architecture of the Fern-leaf is fundamentally dichotomous; 



* Trans. Roy. Soc. Edin., 1916. 



