x INTRODUCTION 
never produced except in cultivation outside the thallus. The 
determination of the genera and species to which they severally 
belong is often uncertain, but their distribution in British 
lichens is somewhat as follows :— 
1. Myxophycez associated with Phycolichenes, many of them 
gelatinous lichens (fig. 3). They are :— 
Scytonema occurring in Thermutis, Spilonema, Lepto- 
gidium, Gyalecta (in part), Placynthium, Polychidium, 
Porocyphus and Coccocar pia. 
Stigonema in Ephebe and Ephebeia. 
Gleocapsa in Euopsis, Pyrenopsis, Synalissa and Psoro- 
tichia. 
Nostoc in Collema, Leptogium, Peltigera, Pannaria and 
Parmeliella. 
Rivularia in Lichina and Pterygium. 
Nylander gave the name gonimia to the blue-green alge of 
the thallus, retaining the term gonidia for the bright-green 
species. The distinction is not generally maintained. 
2, Chlorophycez associated with Archilichenes (fig. 4), as 
follows :— 
Protococcus (Cystococcus, Pleurococcus) and Palmella in 
the greater number of the larger lichens and in many 
crustaceous genera, such as Lecanora, Lecidea, Pertu- 
saria, Verrucaria, etc. 
Trentepohlia in Cenogonium, Dirina, Roccella, Graphi- 
dacex, Pyrenulacee, and also in Thelotrema and 
Gyalecta, rarely in Lecanora and Lecidea. 
Cladophora in Racodium. 
Though as a general rule the alga is less affected than the 
fungus by the symbiotic life, it also may become modified in 
appearance. The blue-green forms may lose their colour, as in 
?leocapsa ; or the strings of cells may be broken up, as occa- 
sionally in Nostoc, Scytonema, etc. Among the Chlorophycez, 
there is occasional change of form both in Protococcus and in 
Trentepohlia ; they revert to their original appearance in free 
cultivation. 
