DEVELOPMENT OF PLANTS 251 



lead to the latter position. However, the fungus has become so 

 dependent upon the algae through ages of association that it 

 does not develop alone naturally and we may therefore consider 

 the lichens as a group coordinate with the fungi. The members 

 of this curious co-partnership are largel}^ ascomycetes and blue- 

 green algae. Numerous species in each of these two groups of 

 plants have become accustomed to living together. The forma- 

 tion 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 



