200 PLANT LIFE 
itself to several kinds of alge, and vice versa. 
In every instance, however, a specific lichen 
results from the union of a definite fungus with 
a particular alga—if either the alga or fun- 
gus be changed, a correspondingly different 
‘* species”? of lichen is formed. 
Both organisms thrive. The algal cells often 
become unusually large, and the fungal 
mycelium is evidently well nourished. But 
multiplication of cells and consequent growth 
is often greatly modified, especially in the 
more specialised lichens in which the two 
organisms become more intimately dependent 
on each other. 
The symbiosis only continues to pay as 
long as the alga is properly exposed to light, 
and for so long as it is properly supplied 
with water, together with the small amount of 
mineral food it requires. The latter offices 
are largely discharged by the fungus, which 
usually attaches the lichen to the substratum, 
whilst its gelatinous walls retain the water 
supplies derived from intermittent showers 
or other sources. Thus a remarkable degree 
of correlation is displayed in the growth 
processes of the specialised lichens, and some 
of them simulate to a wonderful extent the 
form, and partly even the structural arrange- 
ments, to be met with in the green leafy 
shoots of higher plants (Figs. 23a and B). 
Lichens are particularly instructive in 
showing that the form assumed by an organ- 
ism is in the long run determined by the 
