. Morphology of gymnosperms. Gymnosperms; Plant morphology. i88 MORPHOLOGY OF GYMNOSPERMS cotyledon; that the bundles pass from mesarch to exarch while tra- versing the cotyledons; that each cotyledonary bundle gives rise to two protoxylem poles of the root; and that there may be an addition of protoxylem elements after the root structure has been organized. The cycadean type of seedhng is described (42) as consisting of four bundles at the base of each cotyledon and a tetrarch root, which generally becomes diarch below. Miss Thomas (48) has proved. Fig. 213.—Ginkgo biloba: transverse section
. Morphology of gymnosperms. Gymnosperms; Plant morphology. i88 MORPHOLOGY OF GYMNOSPERMS cotyledon; that the bundles pass from mesarch to exarch while tra- versing the cotyledons; that each cotyledonary bundle gives rise to two protoxylem poles of the root; and that there may be an addition of protoxylem elements after the root structure has been organized. The cycadean type of seedhng is described (42) as consisting of four bundles at the base of each cotyledon and a tetrarch root, which generally becomes diarch below. Miss Thomas (48) has proved. Fig. 213.—Ginkgo biloba: transverse section of cotyledon showing mesarch character of vascular bundle; />, protoxylem; *, metaxylem; t, "transfusion tissue"; Xiso.—After Spkecher (46). this cycadean type for the Ginkgo seedling by showing that the appar- ently single bundle that enters the cotyledon is really a plexus of four bundles, separating into two double bundles in the petiole, and each of these separating into its two constituent bundles in the blade. It is in the cotyledons that a very distinct mesarch structure is developed (20). The relation of the number of protoxylem poles in the root to the number of cotyledons (12, 39), that is, a diarch root related to two cotyledons and a triarch root to three, is true only in an indirect way. The original root is tetrarch, and is related to the four cotyle- donary strands from the transition region, and the diarch condition. 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 Coulter, John Merle, 1851-1928; Chamberlain, Charles Joseph, b. 1863; Coulter, John Merle, 1851-1928. Morphology of spermatophytes. Part I. Gymnosperms. Chicago, University of Chicago Press
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