. Nature and development of plants. Botany. 330 GAMETOPHYTE OF FILICALES Several ferns are distinguished by false indusia that are formed by the more or less modified margins of the leaf. In the bracken, Pteridium, the entire membranous margin of the leaf curves over the closely crowded sori (Fig. 226, A), and in the maiden-hair fern, Adiantum, the sporangia are at the ends of the veins and covered by reflexed portions of the leaf (Fig. 226, B). The indusia are lacking in some forms, as in the beech fern, Phegopteris, and in the polypody, Polypodium (Fig. 226, C). These genera are distinguishe
. Nature and development of plants. Botany. 330 GAMETOPHYTE OF FILICALES Several ferns are distinguished by false indusia that are formed by the more or less modified margins of the leaf. In the bracken, Pteridium, the entire membranous margin of the leaf curves over the closely crowded sori (Fig. 226, A), and in the maiden-hair fern, Adiantum, the sporangia are at the ends of the veins and covered by reflexed portions of the leaf (Fig. 226, B). The indusia are lacking in some forms, as in the beech fern, Phegopteris, and in the polypody, Polypodium (Fig. 226, C). These genera are distinguished by the fact, among others, that the leaves of Polypodium drop off, leaving a scar as in our deciduous Fig. 227. The sensitive fern, Onodea: A, portion of normal green leaf. B, a spore-bearing leaf. C, two views of one of the round lobes of B, showing the veins and the sori on inner side of the lobe.—After Bailey. This feature is possibly due to its more or less epiphytic habit. This is the only really xerophytic fern of temperate regions. The sporophyll of the sensitive fern, Onodea, bears a striking outward resemblance to Botrychium, but the sporangia-like bodies are really leaf lobes rolled up and each bears several round sori on its inner side (Fig. 227). In this and several other genera, see Osmunda, the work of photosynthesis is given over to large, green leaves that do not produce sporangia. (&) The Gametophyte.—In the majority of ferns the spore germinates by rupturing the outer coats and producing a germ tube from which one or more delicate rhizoids are cut off. The germ tube elongates, forming a short chain of cells which soon develop by apical growth into a flat thalloid structure, commonly called the prothallium, that is attached to the ground by numer-. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemb
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