Text-book of structural and physiological botany . Fig. 439. — Plnnse of fronds of Ferns with sori ; I. Aspidhun, the sporangiavisible at the margin of the pekate indusium ; II. Aspleniwn : III. Pterisaqzdlina ; IV. the same with the indusium thrown back to show the mode ofattachment of the sporangia; V. Polypodium, indusium wanting ; VI. Hyineno-phylluin or filmy fern ; VII. the same ; the indusium partly removed to show thesporangia (all magnified). often covered by a membranous skin, the indusium [whichis usually an extension of the epidermis] (Fig. 439 i.). In 314 Structural and Physiologi
Text-book of structural and physiological botany . Fig. 439. — Plnnse of fronds of Ferns with sori ; I. Aspidhun, the sporangiavisible at the margin of the pekate indusium ; II. Aspleniwn : III. Pterisaqzdlina ; IV. the same with the indusium thrown back to show the mode ofattachment of the sporangia; V. Polypodium, indusium wanting ; VI. Hyineno-phylluin or filmy fern ; VII. the same ; the indusium partly removed to show thesporangia (all magnified). often covered by a membranous skin, the indusium [whichis usually an extension of the epidermis] (Fig. 439 i.). In 314 Structural and Physiological Botany. most Ferns the sporangia are provided with an elastic ring,the annulus^ running over the apex, which [by its contrac-tion] causes the sporangium to burst [to allow the escape ofthe spores] (Fig. 440 i.); less often it surrounds the sporan-,. Fig. 440. — Sporangia ofFerns; I. Aspidiumy^\\ annulus: with hori-zontal annulus ; III. Os--1711 tnda regalis, royalfern, with incompleteannulus (all greatly magnified). Fig. 441.—Reproduction of a Fern, Pteris serrulata; prothallium proceeding from the spore ; a spore, brhizoids, xy antheridia (x 80) ; II. an archegonium nearthe period of opening, a central cell (x 400); III. theyoung plant springing from the cordate prothallium ; wfirst leaf, r first root (natural size). gium in a horizontal or oblique direction (Fig. 440 11. iii).On germinating, the spore first gives rise to a green thalloid special Morphology and Classification, 315 leaf-like often kidney-shaped structure, lying flat on thesurface of the ground, the prothallium or pro-embryo ( III.), which puts out root-hairs \^hizoids~\ from its underside, and forms the archegonia and antheridia among the former are produced the ciliated antherozoids (); from the latter, wh
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