The structure & development of the mosses and ferns (Archegoniatae) . shapedindusium, 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. Theleaves range from the undivided ones of Vittaria or Scolopen-drium to the repeatedly divided leaves, usually pinnate, of suchforms as Pteris Aqtdlina. In some tropical epiphytic species, such as Asplenium nidus, Platy-cerium, species of Polypodiuin, theleaves are arranged so that theyform receptacles for collectinghumus. In the two latter generathese le
The structure & development of the mosses and ferns (Archegoniatae) . shapedindusium, 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. Theleaves range from the undivided ones of Vittaria or Scolopen-drium to the repeatedly divided leaves, usually pinnate, of suchforms as Pteris Aqtdlina. In some tropical epiphytic species, such as Asplenium nidus, Platy-cerium, species of Polypodiuin, theleaves are arranged so that theyform receptacles for collectinghumus. In the two latter generathese leaves are very much modi-fied, the two forms of leaves beingfamiliar to all botanists in thecommon Platycerimn alcicorne,where the closely overlappinground basal ones are very highlydeveloped.^ The sporangia may almostcompletely cover the backs ofthe sporophylls, as in Platy-ceriuin, or more commonly formdefinite sori, which may or may not have an indusium. Where -^ Goebel, Ueber epiphytische Fame und Muscineen. Ann. diijardin botaniqiie deBtiitenzorg, vol. vii. \.-. Fig. 195.—Pinna from the leaf of Cystopte7-isbulbifcra (Bernh.), with a bud (Ji) at thebase, X 2 ; J, the sori (after Atkinson). XI THE HOMOSPOROUS 377 the latter is present, it is cither formed by the mari^nii (jf llieleaf, as in Adiantuvi or Ptcris, or it may be a special scale-like oiitfjrowth of the lower side of the leaf. In such cases itforms a membraneous covering of characteristic form. Thusin Aspidiuni (Fig. 194, A) it is kidney-shaped, in Aspknhanelongated, and free only along one side. Where, as in Onoclea(Fig. 194, B), the margins of the sporophyll are involute, so asto completely enclose the sori, the indusium is wanting or veryrudimentary. CHAPTER XII LEPTOSPORANGIAT^ HETEROSPORE^ (HYDROPTERIDES)^ The two very distinct families of heterosporous Leptospor-angiatae have obviously but little to do with each other, but,both of them being evidently related to the homosporous forms,they m
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