238 DEVELOPMENT OF LICHENS 



of the lichen and lives upon the alga largely as a parasite and is 

 alone capable of forming reproductive spore bodies would lead 

 to the latter position. The members of this curious co-partner- 

 ship are largely ascomycetes and blue-green algae. Numerous 

 species in each of these two groups of plants have become accus- 

 tomed to living together. The formation of a lichen comes about 

 through the attachment of a hypha of some fungus to one or 

 more algal cells, as shown in Fig. 177, B. In this way, the food 

 manufactured by the algae is absorbed osmotically by the hyphae. 

 In some cases, the fungus obtains its food by means of haustoria 

 which penetrate the algal cells. By the continued growth of the 

 fungus and algae there finally results an interwoven mass of 

 hyphae about the algae. The walls of the fungus are sufficiently 

 transparent to permit the entrance of light and the color of the 

 green algae can readily be seen when the lichen is moistened. 



(a) Structure of the Lichen. — The bulk of the lichen is more 

 usually composed of hyphae which show considerable regularity 

 in their growth and the majority of the algae also are usually 

 confined to a definite zone near the sunned surface of the lichen. 

 Thus, in Fig. 177, A, which represents a cross-section of a lichen, 

 it will be seen that the fungus forms a rather firm layer at the 

 top and bottom of the thallus, while the algae are distributed near 

 the upper surface, where they are exposed to the light, and can 

 therefore carry on photosynthesis. Numerous hyphae projecting 

 from the under surface of the lichen serve to anchor it to the 

 substratum and also assist, doubtless, in the absorption of the 

 earth substances. In some of the gelatinous lichens the fungi 

 and algae are more promiscuously arranged. 



While the fungus is dependent upon the foods manufactured 

 by the algae, the latter are also benefited to an extent by this 

 arrangement since they are protected by the strata of hyphae, and 

 they are also provided with water and crude material which the 

 fungus readily takes up and holds. This mutually helpful rela- 

 tionship of two organisms is termed symbiosis, meaning living 

 together. The strange feature about this co-partnership is the 

 marked change produced in the nature of the two symbionts. As 

 long as they arc independent of each other, very special conditions 



