. Lectures on the evolution of plants. Botany; Plants. THE FERNS 135 the "annulus," a row of cells with thickened walls whose contraction plays an important part in the open- ing of the ripe sporangium and the discharge of the spores (Fig. 35, G, H, r). A „ s _B On comparing the Eusporangiatse and Leptosporan- giatse, one is at once struck by the great disparity in the numbers of the two groups. Prob- ably all living species of Euspo- rangiatse, including the peculiar genus Isoetes, whose posi- tion here is by no means certain, scarcely exceed one hundred, while the Leptosporangiates
. Lectures on the evolution of plants. Botany; Plants. THE FERNS 135 the "annulus," a row of cells with thickened walls whose contraction plays an important part in the open- ing of the ripe sporangium and the discharge of the spores (Fig. 35, G, H, r). A „ s _B On comparing the Eusporangiatse and Leptosporan- giatse, one is at once struck by the great disparity in the numbers of the two groups. Prob- ably all living species of Euspo- rangiatse, including the peculiar genus Isoetes, whose posi- tion here is by no means certain, scarcely exceed one hundred, while the Leptosporangiates, the typical ferns, number probably at least 3500 to 4000 species. In spite of this extraordinary difference in numbers of species, the former group shows much greater range of structure, so much so that it is necessary to make two and perhaps three orders to include them, and the relationships of these are very doubtful. The Lepto-. FiG. 35 (Leptosporangiate Ferns). — A, leaflet of a shield-fern (Aspidium), showing the sori, or sporangial groups, s ; B, a single sorus covered with the kidney-shaped in- dusium, in; C, a fllmy-fern (Triohomanes) with the sorus surrounded by a trumpet- shaped indusium; D, longitudinal section of the sorus, showing the sporangia borne upon the elongated columella; E, F, young sporangia of Polypodium falcatum seen in section; the sporogenous cell is shaded; i, t, the tapetal cells which later are broken down; G, a ripe sporangium of the same species showing the ring or annulus, r, and the stomium, st, where the opening occurs; H, sporangium of a climbing fern (Lygo- sp( m) dlum) with terminal annulus, 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 resemble the original Campbell, Douglas Houghton, 1859-1953. New York, London, The Macmillan company
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Keywords: ., bookcentury1900, bookdecade, booksubjectbotany, booksubjectplants
