Elements of biology; a practical Elements of biology; a practical text-book correlating botany, zoology, and human physiology elementsofbiolog00hunt Year: [c1907] FLOWERLESS PLANTS 173 Stages in the formation of the lichen thallus, showing the rela- tion of the threadlike fungus to the green cells of the alga. After Bornet. flat and irregular. One of the commonest of the lichens (P/i?/scta), found on stone walls or tree trunks, produces cup-shaped bodies from the thallus, in which spores are formed. Most lichens have definite spore-producing structures which protrude from the upper surface of
Elements of biology; a practical Elements of biology; a practical text-book correlating botany, zoology, and human physiology elementsofbiolog00hunt Year: [c1907] FLOWERLESS PLANTS 173 Stages in the formation of the lichen thallus, showing the rela- tion of the threadlike fungus to the green cells of the alga. After Bornet. flat and irregular. One of the commonest of the lichens (P/i?/scta), found on stone walls or tree trunks, produces cup-shaped bodies from the thallus, in which spores are formed. Most lichens have definite spore-producing structures which protrude from the upper surface of the thallus. A lichen is of interest to us chiefly because it shows a partnership to exist between certain green plants, called the algse, and the fungi. A lichen is thus composed of two plants, one at least of which may live alone, but which have formed a partnership for life, and have divided the duties of such life be- tween them. In most lichens the alga, a green plant, forms starch and nourishes the fungus. The fungus, in turn, produces spores, by means of which new lichens are started in life. The body of the lichen is usually pro- tected by the fungus, which is stronger in structure than the green part of the combination. This process of living together for mutual advantage is called symbiosis. Some animals thus combine with plants; for example, the tiny animal known as the hydra with certain of the one-celled algae. Animals also frequently live in this relation to each other. Algae. — The algse are a very diverse collection of plants, con- taining some of the smallest and simplest as well as some of the largest plants in the world. The tiny one-celled Pleuro- coccus is an example of the former; the giant kelps of the Pacific Ocean, which at- tain a length of over one thousand feet, of the latter. The body of the algse is a thallus, which may be plate- like, circular, ribbon-formed, threadlike, or filamentous. It may even be composed of a single cell. A large num- ber
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