THE FUNGI 247 



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 organisms, 

 might be described either under the heading " Algae " 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. 



Most Lichens, however, have also a means of vegetative 

 propagation of their own, which is directly connected 

 with their symbiotic mode of life. It often happens Jaat 

 the tissue of the thallus in Physcia and TTctny other 

 Lichens appears to break up into a powdery mass. 

 This is due to a change in the puidial layer. The 

 algal cells become enveloped, eitHr singly or in little 

 groups, by a luxuriant gro^tn of the fungal hyphse 

 among which they lie. J-'he hyphse surrounding each 



