. The anatomy of woody plants. Botany -- Anatomy. 366 THE ANATOMY OF WOODY PLANTS by the accompanying figure (Fig. 264) of the stem of a seedling collected by Professor Thompson in Southwest Africa. It is clear that the same polydesmic organization of the axis is present as is found in the older stem of species of Gnetum. In the South African genus, however, the polydesmic condition extends to the roots and is accordingly to be regarded as a more innate feature of organization than in Gnetum. The structure of Welwitschia suggests a climbing ancestry. Although we do not know the explanation of
. The anatomy of woody plants. Botany -- Anatomy. 366 THE ANATOMY OF WOODY PLANTS by the accompanying figure (Fig. 264) of the stem of a seedling collected by Professor Thompson in Southwest Africa. It is clear that the same polydesmic organization of the axis is present as is found in the older stem of species of Gnetum. In the South African genus, however, the polydesmic condition extends to the roots and is accordingly to be regarded as a more innate feature of organization than in Gnetum. The structure of Welwitschia suggests a climbing ancestry. Although we do not know the explanation of the polydesmic condition in climbers, it is clearly co-ordinated with the vine habit. We may suppose that the forbears of the genus were originally forest climbers and that the surviving strongly truncated desert species has per- sisted in its present habitat with the re- tention of the ances- tral polydesmy. A similar suggestion has been made in an earlier chapter as a possible explanation of the phenomenon of polydesmy in the cycadean forms, living and extinct. Having considered the general topography of the stem in the Gnetales with particular reference to the presence of large rays of the dicotyledonous type and the phenomenon of polydesmy, we may now profitably turn our attention to the more minute organ- ization of the wood. The ligneous tissues consist of rays and longitudinal elements. The former have already been sufficiently discussed in previous paragraphs. The longitudinal structures of the xylem consist of tracheids, vessels, and storage parenchyma. The tracheids do not need any extended reference, as they have. FIG. 264.—Transverse section of young stem of Welwitschia 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 Jeffrey, Edward C. (Edward Charles), b. 1866. Chicago, Ill. , The
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