COMPOUND ORGANISMS 199 



waste products formed as the result of the 

 chemical life processes of its associate. Such 

 an association is often spoken of as symbiosis, 

 but it is evident that the transition from 

 symbiosis to parasitism is only a matter of 

 degree. An excellent example of symbiosis is 

 furnished by Lichens. These plants are com- 

 pound organisms, made up of a fungus on 

 the one hand, and a green alga on the other. 

 It is often possible to separate the two, and 

 to cultivate them apart, and the habit of 

 growth (except in the most primitive forms) 

 is very different from that which occurs 

 when they associate to form the lichen. 

 Lichens are formed in countless numbers 

 every spring, and scrapings from the bark of 

 damp trees at this season will generally yield 

 quite a large selection of these compounded 

 organisms in the making. Sometimes a 

 particular fungus filament which comes in 

 contact with an appropriate alga may be seen 

 to branch and then to embrace the alga 

 within its threads. Many of these early 

 beginnings of lichens are really due to the 

 escape, from older lichens, of algal cells, each 

 of which is already accompanied by a few 

 fungal hyphse. These young associations 

 called soredia may be recognised as the green 

 or grey powdery dust which often occurs on 

 lichens when in vigorous growth. 



It is possible to make a lichen artificially, 

 by bringing together the alga and fungus. 

 And we learn that one fungus may attach 



