376 



MOSSES AND FERNS 



CHAP. 



Among the many genera and species aside from these, while 

 there is extraordinary variety, the differences are all of 

 secondary importance, and consist mainly in the form and 



hy\ 



Fig. 194.— a, Pinnule of Aspidmm spiniilosuin (Schw.), showing the sori (s) with kidney-shaped 

 indusium, X4; B, cross-section of a pinna from a young sporophyll of Onoclea struthioj/teris ; 

 s, sorus, X40. 



venation of the leaves and the position of the sporangia. The 

 leaves range from the undivided ones of Vittaria or Scolopen- 

 drium to the repeatedly divided leaves, usually pinnate, of such 

 forms as Pteris Aqtdlina. In some tropical epiphytic species, 



such as Asplenium nidus, Platy- 

 cerium, species of Polypodiuin, the 

 leaves are arranged so that they 

 form receptacles for collecting 

 humus. In the two latter genera 

 these leaves are very much modi- 

 fied, the two forms of leaves being 

 familiar to all botanists in the 

 common Platycerimn alcicorne, 

 where the closely overlapping 

 round basal ones are very highly 

 developed.^ 



The sporangia may almost 

 completely cover the backs of 

 the sporophylls, as in Platy- 

 ceriuin, or more commonly form 

 definite sori, which may or may not have an indusium. Where 



-^ Goebel, Ueber epiphytische Fame und Muscineen. Ann. diijardin botaniqiie de 

 Btiitenzorg, vol. vii. 



\.- 



Fig. 195. — Pinna from the leaf of Cystopte7-is 

 bulbifcra (Bernh.), with a bud (Ji) at the 

 base, X 2 ; J, the sori (after Atkinson). 



