LICHENS AND THE ROCK FLORA. 239 



the sun's rays that the stone becomes almost too 

 hot to touch. 



And yet it is upon such boulders that lichens are 

 found in profusion. The fungus forms a sort of 

 nest or home in which the algal cells are enclosed. 

 It also absorbs water and such nitrogenous and 

 mineral matters as are necessary, whilst the alga 

 obtains the sunlight-energy and carbon which are 

 essential to the life of both. There has been much 

 controversy as to the exact degree in which the two 

 co-operate. Such words as coininensalism, consortisvi, 

 symbiosis^ kelotisin, have been invented and defined, 

 and have also been shown to be unsuitable and inexact 

 by many writers. It is, however, quite clear that 

 the life of an ordinary lichen is not possible unless 

 both members are present, and that the fungus and 

 the alga are absolutely dependent on one another. 

 It is not difficult to grow the fungus by itself in an 

 artificial nourishing solution, and with a little trouble 

 the algal cells can be separated from the lichen and 

 will develop freely, if they are given all they require. 

 But neither could exist alone in the places where 

 the lichen thrives, and thus, as regards lichens, the 

 controversy is quite unnecessary. 



The structure of the lichen is simple, and yet is 

 exceedingly well suited to its position. The fungus 

 part consists of a series of fungal threads or hyphae. 

 The large leaden-brown lichen Peltigera canina, common 

 in woods on moss, is a fair example of the general 

 structure. The underside of this lichen is white, 

 and covered with an irregular projecting network of 

 yellowish-white veins, bearing at intervals distinct 

 brown or whitish hairs, or rhizines. These rhizines 

 attach themselves very firmly to the mosses, dead 



