. Principles of modern biology. Biology. 210 - Multicellular Plants do not adhere to any standard type of life cycle. The commonest type of cycle is ex- hibited by Spirogyra (p. 168), and Ocdo- gonium (Fig. 12-8). In both these algae, the colony is composed of haploid cells; and any cell of the filament can give rise to one or more gametes. Accordingly, these plants are equivalent to the gametophyte generation in the higher plants. However, scarcely any sporophyte generation can be recognized, be- cause the zygote divides meiotically as soon as it begins to germinate, and all the cells of the
. Principles of modern biology. Biology. 210 - Multicellular Plants do not adhere to any standard type of life cycle. The commonest type of cycle is ex- hibited by Spirogyra (p. 168), and Ocdo- gonium (Fig. 12-8). In both these algae, the colony is composed of haploid cells; and any cell of the filament can give rise to one or more gametes. Accordingly, these plants are equivalent to the gametophyte generation in the higher plants. However, scarcely any sporophyte generation can be recognized, be- cause the zygote divides meiotically as soon as it begins to germinate, and all the cells of the new filament are haploid (Fig. 12-8). The opposite type of cycle is found in Fucus (Fig. 12-9) and some other brown algae. The body cells of this multicellular seaweed are all diploid, and only the gametes are haploid. The sperm and eggs respectively are formed by meiosis in hollow structures found at the ends of many of the branches of the sexually mature plants. After fertilization, which occurs when the gametes are extruded into the sea water, the diploid zygote multi- plies mitotically, forming all the cells of the new individual. An irregular alternation of generations, foreshadowing the reproductive habits of higher plants, occurs in a few Thallophyta— as in the bread mold, Rhizopus (p. 211). The spores of the mold are haploid, and in a suitable medium, such as moist bread, each haploid spore grows into a mycelium, called. Fig. 12-9. Rockweed (Fucus) growing on intertidal rocks along the California coast. (From Trie Plant World, by Fuller and Carothers. Holt, Rinehart and Winston, Inc.). 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 Marsland, Douglas, 1899-. New York, Holt, Rinehart and Winston
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