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 hyphe. 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 
