Fam. Rivulariaceae. Trichomes with a heterocyst at the base and tapering upwards, enclosed in mucilage ([Fig. 21]).
Fig. 20. Stigonema sp. × 200 (after Comère).
Fig. 21. Examples of Rivularia; A, B, C, R. Biasolettiana Menegh.; D and E, R. minutula Born. and Fl. A and D nat. size; B, C and E × 480 (after West).
Rivularia Thuret. In tufts fixed at the base and forming roundish gelatinous colonies; colour, blue-green. The gonidium of Lichinaceae has been identified as R. nitida Ag.
Algae belonging to one or other of these genera of Myxophyceae also combine with the hyphae of Archilichens to form cephalodia[279] and Krempelhuber[280] has recorded and figured a blue-green alga, probably Gloeocapsa, in Baeomyces paeminosus from the South Sea Islands. They also form the gonidia in a few species and genera of such families as Stictaceae and Peltigeraceae.
b. Chlorophyceae associated With Archilichens. The lichens of this group are by far the most numerous both in genera and species, though fewer algal families are represented.
Fam. Protococcaceae. Consisting of globular single cells, aggregated in loose colonies, dividing variously.