INTRODUCTION 



Most of the lichens of the temperate regions of the United States belong 

 with the Ascomycetes. They have become so modified, in many instances, since 

 entering into relationship with the alga that there is little resemblance between 

 them and the ancestral forms. There is sufficient likeness, however, to lead to a 

 general conviction that part of them have evolved from the Discomycetes and 

 part from the Pyrenomycetes. Of these two groups such genera as Peziza, Patel- 

 laria, Hysterographium, Phacidium, Chaetomium, and Sordaria are among those 

 which may be closely related to the ancestors of the lichens. In the subtropical 

 regions of southern United States are found two genera which belong to the 

 Hymenolichenes. Their relationship apparently is with the fungi of the family 

 Thelephoraceae. The imperfect lichens are represented by one genus in which no 

 spore form has been discovered. This has been retained in the imperfects although 

 many workers have placed it among the Ascolichenes. 



The algal hosts are much better understood than the fungi. Indeed, in the 

 description of genera, statements are made regarding the algae to be found in 

 each genus, and some use is made of these algal types in classification into 

 families and genera. The algae most commonly found in the lichen thalli belong 

 to the genus Cystococcus (Fig. 1 a), and this genus is closely related to Pleuro- 

 coccus (Fig. 2 d), which is supposed to be the algal host of a few common lichens. 



Figure 1 

 Algal types with associated fungal hyphae. a, Cystococcus, each cell surrounded by 

 haustoria. b, Trentepohlia, showing a branching chain of cells, surrounded by haustorial hyphae. 

 c, Nostoc, showing the algal filaments and the intermingled fungal hyphae. d, Gloeocapsa, 

 showing the haustoria and the fungal hyphae surrounding the algal cells, a, enlarged 750 

 diameters; b, 32 5 diameters; c, 500 diameters; d, 650 diameters. From Schneider. 



