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 of the algse inhaliit the 

 water, either fresh or salt. 

 In color they vary from green through the shades of blue-green to 

 yellow, brown, and red. These colors are best seen in the sea- 

 weeds, all of which, however, contain chlorophyll. In the red 



A red seaweed, showing a finely divided 

 thallus body. 



