. Heredity and evolution in plants . independence to parasitism(mistletoe and dodder), or to saprophytism (Indian pipeand toad-stools). The thirty odd species of the Duckweed family, relatedto the Arum family (Jack-in-the-pulpit, calla, skunk cab-bage, sweet flag, etc.), illustrate regression; they comprisethe simplest, and some of them the smallest of all floweringplants. The plant body of Lemna is a tiny disc-shaped,thallus, having a central vein (vascular strand) with orwithout branches. Each plant has one root with novascular tissue. The flowers, borne on the margin orupper surface of the
. Heredity and evolution in plants . independence to parasitism(mistletoe and dodder), or to saprophytism (Indian pipeand toad-stools). The thirty odd species of the Duckweed family, relatedto the Arum family (Jack-in-the-pulpit, calla, skunk cab-bage, sweet flag, etc.), illustrate regression; they comprisethe simplest, and some of them the smallest of all floweringplants. The plant body of Lemna is a tiny disc-shaped,thallus, having a central vein (vascular strand) with orwithout branches. Each plant has one root with novascular tissue. The flowers, borne on the margin orupper surface of the thallus, have one simple pistil andonly one stamen (Fig. 44). The dozen or more species EVOLUTION 8l of Wolffia possess still simpler bodies, somewhat globose,with neither roots, veins, nor other organs, except flowers;even flowers are unknown in some species (, Wolffiapopulifera, Fig. 44). Wolffia punctata measures mm. long. The plants are fittingly described inthe manuals as minute, alga-like grains, floating on or. FIG. 44.—Lemnacese. a, b, c, Lemna Irisidca; d, Woljfia punclata;e, f, Wolffia papnlifcra. (Redrawn from Brill on and Brown, slightlymodified.) just beneath the surface of still water. Some botanistsconsider the plant body as morphologically a frond, othersas a leafless stem. Since the first plant-body from the seedis only a matured cotyledon, or seed-leaf, Goebel considersthat it cannot be interpreted as other than a free-livingleaf. These tiny, simple plants are considered to have X2 IIKKKDITV AND EVOLUTION IX JM. \MS originated by regressive evolution, theiv simplificationbeing closely correlated with a reversion from dry land toan aquatic habit of life. A similar reduction of structureis found in the tiny floating ferns, Sahinia and Azolla. 71. Inorganic Evolution.—The process of evolution isnot confined to living things, but, as indicated above,applies to all nature. Even the chemical elements arebelieved to have been produced by evolution
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