. Heredity and evolution in plants. Heredity; Plants. THE EVOLUTION OP PLANTS 229 habit. They are therefore in all probability adaptations to that habit. Two more—stem anatomy and the ap- parently terminal cotyledon in the embryo—may be considered as direct consequences of such adaptations; the stem anatomy acquiring its peculiar features from the insertions of numerous broad-based leaves on a squat subterranean axis, and the embryonic cotyledonary number arising from the congenital fusion of two ancestral cotyle- dons. The seventh character—trimerous floral symmetry—bears no obvious re- latio
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. Heredity and evolution in plants. Heredity; Plants. THE EVOLUTION OP PLANTS 229 habit. They are therefore in all probability adaptations to that habit. Two more—stem anatomy and the ap- parently terminal cotyledon in the embryo—may be considered as direct consequences of such adaptations; the stem anatomy acquiring its peculiar features from the insertions of numerous broad-based leaves on a squat subterranean axis, and the embryonic cotyledonary number arising from the congenital fusion of two ancestral cotyle- dons. The seventh character—trimerous floral symmetry—bears no obvious re- lation to the geophilous habit, but is not inconsistent with it." Recent evidence as to how monocoty- ledony may have been derived from dicotyledony has been furnished by a study of the embryogeny of Agapanthus umbellatus L'Her (Fig. 108), a South African plant of the Lily family. The sequence of events is as follows.^ -"oj^ocotyledonous ^ embryo. B, aicoty- As the massive pro-embryo enlarges the ledonous embryo, r0ot-end • elongates, thus remaining (Redrawn from narrow and pointed; while the shoot-end P^°JJ^^ ^- J" ^• widens, becoming relatively broad and flattish. At this broad and fiat end the peripheral cells remain in a state of more active division than do the central cells, and form what is known as the cotyledonary zone. In this zone two more active points (frimordia) appear and begin to develop. Soon the whole zone is involved in more rapid growth, resulting in a ring or ' The above description closely follows Coulter and Land. The origin of monocotyledony. Bot. Gaz. 57: s°9-5i8, June, 1914-. Fig. 108.— Agapanthus umbellatus. A, . 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 work.. Gager, C. Stuart (Charles Stuart), 1872-1943. Philadelphia, P. Blakiston's Son & Co.