SAPROPHYTISM AND SYMBIOSIS 331 



mutual advantage, is admirably illustrated by lichens. 

 These plants grow commonly on the trunks and stems of 

 trees, on old boards and fences, and on rocks (Fig. 237). 

 The plant body is a thallus, and, when its inner structure 

 is examined, it is seen to be a composite plant, formed by 

 a species of green alga, resembling Pleurococcus, and sur- 

 rounded by the mycelium of a filamentous fungus (Fig. 

 238). If supplied with suitable moisture, the alga can 

 live alone, because it has chlorophyll, but the fungus, 

 not having chlorophyll, cannot live alone. By uniting 

 into a common body, each plant supplies what the other 

 needs. 



The fungal portion of lichens reproduces by means of 

 spores borne in asci, and is therefore an ascomycete. The 

 apothecium (" fruiting" portion of the lichen) is in reality 

 a modified ascocarp (Fig. 238). In some species the 

 apothecia occur at the summit of specialized, upright 

 branches or podetia (Fig. 239). Only a few years ago 

 the interesting discovery was made that lichens may be 

 experimentally produced by the artificial union of certain 

 algae and fungi (Fig. 240). Some of the lichens thus pro- 

 duced resembled those found in nature, while other com- 

 binations were entirely new. 



An interesting case of the symbiotic association of four 

 genera, if not to their mutual benefit, at least without 

 apparent detriment to either, is found in the roots of some 

 of the Cycadaceae. All the genera of this family produce 



alone, cannot do this. 2, Longitudinal section of apothecium; a, thecium; 

 b and c, the two layers of the hypothecium; d, upper algal layer; e, colonies 

 of algae distributed through the medullary layer; /, lower algal layer; g, 

 lower cortical layer. 3, Cross-section of vegetative portion of thallus. 4, 

 Paraphyses (sterile fungal filaments), and spore-sac (ascus), containing 

 ascospores. 5, Ascospores. 6, Algal cells, surrounded by fungal hyphae 

 with haustoria (absorbing branches). (After Schneider.) 



