2 LICHEN FLORA OF THE UNITED STATES 



In the genera Collema, Synechoblastus, Leptogium, and some others the algal host 

 is Nostoc (Fig. 1 c). Trentepohlia (Fig. 1 b) is found associated with a con- 

 siderable number of crustose lichens. Other algae which occur only rarely are 

 Gloeocapsa (Fig. 1 d), Rivularia (Fig. 2 a), Polycoccus (Fig. 2 b), Dactylococcus 

 (Fig. 2 c), and Sirosiphon (Fig. 2 e). 



Figure 2 



Algal types with associated fungal hyphae. a, Rivularia with the fungal hyphae inter- 

 mingled, b, Polycoccus: colonies of cells surrounded by hyphae and haustoria. c, Dactylococcus: 

 elliptical algal cells and fungal hyphae. d, Pleurococcus, the irregular algal cells inclosed in 

 fungal hyphae. e, Sirosiphon, a terminal branch not showing the fungal hyphae. a, much 

 enlarged; b, c, enlarged 325 diameters; d, 450 diameters; e, 250 diameters. From Schneider. 



HYMENOLICHENES 



The Hymenolichenes are those which produce their spores on basidia. The 

 thallus or vegetative tissue spreads irregularly over the substratum or becomes 

 pileate. It is composed of interwoven hyphae and shows little or no differentiation 

 into layers. On the under surface of the thallus is produced the hymenium, which 

 is composed of basidia and paraphyses. Each basidium produces 4 sterigmata on 

 the tips of which are borne hyaline or brownish spores. 



LICHENES IMPERFECTI 



Lichens in which no spore form has been discovered have been placed in this 

 group. The thallus is composed of a more or less densely woven mass of hyphae 

 often giving a powdery appearance to the substratum. 



ASCOLICHENES 

 The Ascolichenes are those which produce their spores in sacs or asci. To this 

 group belong most of the lichens of the United States. They are discussed in 

 detail in the following pages. 



GROSS MORPHOLOGY 



The Thallus. The fundamental part of the lichen is the vegetative tissue or 

 thallus. This thallus may be erect, rising from the substratum; pendulous, hanging 

 downward from it; conspicuous or inconspicuous and flat, closely or loosely attached 

 to the substratum ; or inconspicuous and largely or even wholly imbedded in the sub- 

 stratum. Erect and pendulous forms are commonly called fruticose thalli, and 



