*4 MOUNTAIN AND MOORLAND 



green dust that spreads on tree-stems in damp weather, 

 and (2) of a colourless plant or Fungus, like many a 

 mould. The Fungus part supplies the water and 

 soluble salts from the rocks; the Alga part is able to 

 build up organic compounds with the help of the sun- 

 shine. Thus the two partners work into one another's 

 hands. This is what is called symbiosis a mutually 

 beneficial internal partnership between two living 

 creatures of quite different kinds. The Alga partner 

 can be got to live away from its Fungus partner, but 

 the Fungus can very rarely do without the Algae. The 

 master can seldom live without his green slaves. One 

 of the discoverers of this partnership, Professor 

 Schwendener, wrote : " As the result of my researches, 

 all these growths (lichens) are not simple plants, not 

 individuals in the ordinary sense of the word ; they are 

 rather colonies consisting of hundreds and thousands 

 of individuals, among which, however, one predomin- 

 ates, while the rest in perpetual captivity prepare the 

 nutriment for themselves and their master. This 

 master is a fungus, a parasite which is accustomed to 

 live upon others' work; its slaves are green Algae, 

 which it has sought out, or indeed caught hold of and 

 compelled into its service. It surrounds them, as a 

 spider its prey, with a fibrous net of narrow meshes, 

 which is gradually converted into an impenetrable 

 covering; but while the spider sucks its prey and 

 leaves it dead, the Fungus incites the Algae found in 

 its net to more rapid activity indeed, to more vigor- 

 ous increase." 



It is very interesting to know that some lichens can 

 be made artificially, by taking a known Alga and sow- 

 ing a known Fungus upon it. A known lichen may 



