THE FUNGI 247 



places, as, for example, on the surface of exposed rocks, 

 where they could not possibly carry on their life if left 

 to themselves. 



Such a relation between two organisms which live in 

 common, and perform certain functions each for the 

 good of the other, is known by the name of symbiosis, or 

 commensalism, the former word simply calling attention 

 to their living in union, while the latter term means 

 that they share the same table, implying that they 

 mutually help each other to food. 



2. EEPRODUCTION. 



The Lichens being, as we have seen, compound organ- 

 isms, might be described either under the heading " Algge " 

 or " Fungi." It is usual, however, to take them with 

 the latter class, because the organs of fructification, on 

 which classification is chiefly based, belong entirely to 

 the fungal partner. The captive Algae go on increasing 

 by division, but rarely produce any characteristic repro- 

 ductive organs, so long as they form part of the Lichen. 

 Nearly all Lichen -Fungi are Ascomycetes, and the 

 majority belong to the group Discomycetes, in which the 

 hymenium is exposed to the air when mature. So far as 

 the fructification is concerned, there is no essential differ- 

 ence between Lichens and other Fungi of the same group, 

 which lead an ordinary parasitic or saprophytic existence. 



The ascus-fruits of Physcia parietina are conspicuous 

 to the naked eye as flat, shield-like discs on the upper 

 surface of the thallus, generally of a rather deeper 

 orange colour than the rest of the plant (see Fig. 98). 

 These open fruits of the Discomycetes bear the name of 

 apothecia. In the mature condition there is a rim of 



