Page:An introduction to physiological and systematical botany (1st edition).djvu/517

 Rh of which some of the latter have nevertheless a spurious vestige. All the former, and some of the latter, are dorsiferous, bearing fruit on the back of the frond, and of these the fructification is either naked, or else covered with a membranous involucrum. The genera are distinguished by Linnæeus according to the shape and situation of the spots, or assemblages of capsules, besides which I have first found it necessary to take into consideration the absence or presence of the involucrum, and especially the direction in which it bursts. See Tracts relating to Nat. Hist. 215. t. 1.

Polypodium, ''Engl. Bot. t. 1149, has no involuerum; Aspidium, t. 1458—1461, has a single, and Scolopendrium, t.'' 1150, a double one. Osmunda, t. 209, has been remarked by Professor Swartz to have a spurious ring. It is one of those ferns the lobes of whose frond are metamorphosed, as it were, into spikes of capsules. Botrychium of Swartz, more distinctly spiked, and having no vestige of a ring, is separated by him from Osmunda. See one species of it in ''Engl. Bot. t.'' 318.