. Nature and development of plants. Botany. 324 STRUCTURE OF THE STEM entiated into an epidermis, stomata, chlorenchyma and vascular bundles as in the higher plants (Fig. 223). The stems are more usually prostrate, creeping rhizomes that branch sparingly and so gradually give rise to colonies of ferns. One of the most attractive features of certain tropical districts is the tree ferns with erect stems of palm-like appearance which lift their great crowns of leaves 30 to 50 feet in the air. The vascular bundles are more commonly of the concentric type (Fig. 221). Perhaps we should repeat that a
. Nature and development of plants. Botany. 324 STRUCTURE OF THE STEM entiated into an epidermis, stomata, chlorenchyma and vascular bundles as in the higher plants (Fig. 223). The stems are more usually prostrate, creeping rhizomes that branch sparingly and so gradually give rise to colonies of ferns. One of the most attractive features of certain tropical districts is the tree ferns with erect stems of palm-like appearance which lift their great crowns of leaves 30 to 50 feet in the air. The vascular bundles are more commonly of the concentric type (Fig. 221). Perhaps we should repeat that a bundle is composed of two types of elon- gated cells, thick walled woody cells (the xylem) and thin walled. Fig. 222. Arrangements of the sporangia: A, lobe of leaf of Dryopteris with sporangia grouped in circular sori, s. B, sorus enlarged, showing the shield-like membrane or indusium, in, covering the sporangia, sp. C, lobe of leaf of Asplenium with elongated sori, s. cellulose cells (the phloem). In the fern type of concentric bundle the phloem surrounds the xylem. These bundles are united in the stem in a variety of ways. In the simplest case we find a central core of xylem surrounded by phloem which gives off strands or vascular bundles to the leaves and roots. In the majority of ferns this core of xylem is modified by the develop- ment of rather delicate pith-like cells in its center, consequently it appears as a ring in cross section or as a cylinder in longitudinal view. This ring is broken into segments owing to the fact that. 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 Curtis, Carlton Clarence, 1864-1945. New York, H. Holt
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