Nature and development of plants . ; cr,cortical region. tures of certain tropical districts are the tree ferns with erectstems of palm-like appearance which lift their great crowns ofleaves 30 to 50 feet in the air. The vascular bundles are of 294 SPORANGIA OF FILICALES the concentric type (Fig. 221) and usually form a hollow net-work in the stem (Fig. 220, x). Certain cells of the cortex andpith often become modified into strengthening cells of stereome(Fig. 221, st). Roots arise near the base of the leaves, and insome of the tree ferns form a thick mat-like covering on thestems. They origin
Nature and development of plants . ; cr,cortical region. tures of certain tropical districts are the tree ferns with erectstems of palm-like appearance which lift their great crowns ofleaves 30 to 50 feet in the air. The vascular bundles are of 294 SPORANGIA OF FILICALES the concentric type (Fig. 221) and usually form a hollow net-work in the stem (Fig. 220, x). Certain cells of the cortex andpith often become modified into strengthening cells of stereome(Fig. 221, st). Roots arise near the base of the leaves, and insome of the tree ferns form a thick mat-like covering on thestems. They originate from the endodermis of the bundles andpossess a root cap and radial arrangement of the vascular bundlesas in higher plants. (a) Structure and Character of the Sporangia.—The spor-angia, instead of being produced in the tissues of special branchesas in Ophioglossum, are borne in curiously constructed capsules,usually situated on the under surface of the ordinary greenleaves (Fig. 222). The sporangia-bearing leaves are usually. Fig. 222. Arrangement of the sporangia: A, lobe of leaf of Dryopteriswith sporangia grouped in circular sori, s. B, sorus enlarged, showing theshield-like membrane or indusium, in, covering the sporangia, sp. C, lobeof leaf of Asplenium with elongated sori, j. called sporophylls, meaning spore-bearing leaves. In some casesthe sporophylls arc highly modified, being entirely given up tospore production and therefore quite different from the greenleaves (Fig. 227). The sporangia are usually associated ingroups or sori (sing, sorus) on the vascular bundles and pro- DEVELOPMENT OF PLANTS 295 tected by a membranous outgrowth of the epidermis, knownas the indusium (Fig. 222, B). Each sporangium originatesusually from a single epidermal cell, which by repeated divisions
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