THALLOPHYTA: FUNGI 



155 



by fission within the lichen body. The fungous components produce 

 ascocarps, generally in abundance (Fig. 127). These are either apothecia 

 or perithecia. Sex organs have been observed in many lichens. The 

 ascogonium is a spirally coiled multicellular filament commonly terminat- 

 ing in a trichogyne. The male cells, or spermatia, are borne on branching 

 hyphae arising within a flask-like chamber, or spermogonium. After 



Fig. 126. Cross section through the body of a lichen (Physcia), showing cells of the alga 

 (shaded) surrounded by a mass of interlacing fungous hyphae, X 500. 



fertilization, which may not always take place, the ascogonium gives rise 

 to many ascogenous hyphae and paraphyses (Fig. 127 B). At the tips of 

 the ascogenous hyphae typical asci with eight ascospores are formed. An 

 ascospore, in germination, produces hyphae that die unless they come in 

 contact with a suitable alga. 



The relation of the two lichen components to each other is important to 

 understand. The fungus lives on the alga as a parasite but does not kill 

 it. In fact, the alga seems to be only slightly injured, merely sacrificing 

 some of the food that it makes. At the same time, however, the alga is 

 benefited in that the fungous body readily absorbs and retains moisture, 



