[Chap. XLIV THE FUNGI 565 



still others are much branched and cushion-like (fruticose). The latter 

 are represented by the so-called "reindeer moss" (Cladonia) of north 

 temperate and tundra regions. Lichens are usually grayish-green in color, 

 but many species are bright yellow, red, brown, or black. 



A lichen is sometimes considered to be merely a fungus that is para- 

 sitic on certain algae, just as fungi are parasites on wheat, corn, and 

 potatoes (Fig. 255). Experiments have shown that the fungus of the 



Fig. 255. Early stages in the development of a lichen. The hyphae surround and 

 penetrate the cells of the alga. After Bonnier. 



lichen, when growing on culture media, usually does not develop a body 

 structure resembling that of the lichen; nor does the alga in pure culture. 

 Nevertheless, the form of the lichen is heritable and reproduced from 

 one generation to another. For this reason it seems desirable to regard 

 the lichens as distinct compound plants and to classify them as a separate 

 group. There can be no doubt that the fungus derives most of its food 

 from the algae enclosed by it. The alga in turn may have a more constant 

 water supplv because of its covering of fungous hyphae. The algae have 

 a much longer photosynthetic season inside the lichen than if they grew 

 directly on the same substrate. 



The species of fungi of the lichens usually belong to the group pro- 

 ducing asci and ascospores. A few species of basidiomycetes are also 

 known to form a part of the hchen body. The alga is either a green or a 

 blue-green; one of the commonest is Pleurococctis ( see Chapter XLVII ) . 

 Vegetative propagation is exceedingly common among lichens, and takes 



