392 



GENERAL BIOLOGY 



Lichens appear as single organisms. They were long so 

 considered. It is convenient to describe them still as single 

 species; for they are such, for all practical purposes. But 

 they are composite species, each consisting of a fungus and 

 an alga, living together in structural and physiological 

 union. 



The form of 

 the combina- 

 tion is domina- 

 t e d by the 

 fungus, which 

 develops an un- 

 derlying strat- 

 um for attach- 

 ment to the 

 support, and a 

 covering cortical 

 layer having 

 great capacity 

 for resisting 

 e^'aporation — of 

 great advantage 

 in exposed situ- 

 ations : and in 

 its more porous 

 open fi b r o u s 

 middle layer, 

 shelters a host 

 of algal cells. 

 The color of the 

 latter shows 

 through when the lichen is wet, but the true relations 

 of parts are best made out by cutting vertical sec- 

 tions (fig. 229), through the thallus, and examining them 



Fig. 228. A strap lichen growing on a tree trunk in 

 damp woods. 



