Elementary botany . mesh of colorless threads which in certain definite portionscontain entangled green cells. The colorless threads are those of the fungus,while the green cells are those of the alga. These green cells of the alga per-form the function of chlorophyll bodies for the dual organism, while the threadsof the fungus provide the mineral constituents of plant food. The alga, HOW PLANTS OBTAIN FOOD. 95 while it is not killed in the embrace of the fungus, does not reach the per-fect state of development which it attains when not in connection with thefungus. On the other hand the fungu
Elementary botany . mesh of colorless threads which in certain definite portionscontain entangled green cells. The colorless threads are those of the fungus,while the green cells are those of the alga. These green cells of the alga per-form the function of chlorophyll bodies for the dual organism, while the threadsof the fungus provide the mineral constituents of plant food. The alga, HOW PLANTS OBTAIN FOOD. 95 while it is not killed in the embrace of the fungus, does not reach the per-fect state of development which it attains when not in connection with thefungus. On the other hand the fungus profits more than the alga by thisassociation. It forms fruit bodies, and perfects spores in the special fruitbodies, which are so very distinct in the case of so many of the species ofthe lichens. These plants have lived for so long a time in this close associa-tion that the fungi are rarely found separate from the algae in nature, but ina number of cases they have been induced to grow in artificial cultures sep-. Fig. (peltigera), section of thallus ; dark zone of rounded bodies made up largely of thealgal cells. Fungus cells above, and threads beneath and among the algal cells. irate from the alga. This fact, and also the fact that the algae are oftenfound to occur separate from the fungus in nature, is regarded by many as anindication that the plant body of the lichens is composed of two distinct or-ganisms, and that the fungus is parasitic on the alga. 201. Others regard the lichens as autonomous plants, that is, the two or-ganisms have by this long-continued community of existence become unifiedinto an individualized organism, which possesses a habit and mode of life 96 PHYSIOLOGY distinct from that of either of the organisms forming the component community of existence between two different organisms is called bysome mutualism, or symbiosis. While the alga inclosed within the meshesof the fungus is not so free to develop, and probably does not atta
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