INTRODUCTION xi 
Fic. 4.—Bright-green alge that form gonidia. a, Protococeus ; 
b, Palmella ; c, Trentepoklia ; d, Cladophora. (a, b, x 250, 
aiter Hassal ; c, after Cooke, x 250; d, after West, x 60.) 
MORPHOLOGY. 
There are three principal types of development in the 
vegative body or thallus of lichens :— 
1. Crustaceous, or encrusting lichens which spread over trees, 
rocks or soil, with a more or less well-developed crust varying in 
thickness, form and colour. The thallus consists generally of an 
upper cortical layer of hyphe; beneath the cortex a zone of 
algal cells and lower still a medulla of fungal filaments, the latter 
resting directly on the substratum ; sometimes the crust is only 
