a. Myxophyceae associated with Phycolichens. The blue-green algae are characterized by the colour of their pigments which persists in the gonidial condition giving various tints to the component lichens, and by the gelatinous sheath in which most of them are enclosed. This sheath, both in the lichen gonidia[277] and in free-living forms, imbibes and retains moisture to a remarkable extent and the thallus containing blue-green algae profits by its power of storing moisture. Myxophyceae form the gonidia of the gelatinous lichens as well as of some other non-gelatinous genera. Several families are represented[278]:
Fam. Chroococcaceae. This family includes unicellular algae with thick gelatinous sheaths. They increase normally by division, and colonies arise by the cohesion of the cells. Several genera form gonidia:
1. Chroococcus Naeg. Solitary or forming small colonies of 2-4-8 cells ([Fig. 16]) generally surrounded by firm gelatinous colourless sheaths in definite layers (lamellate). Chroococcus is considered by some lichenologists to form the gonidium of Cora, a genus of Hymenolichens.
2. Microcystis Kütz. Globose or subglobose cells forming large colonies surrounded by a common gelatinous layer (gonidia of Coriscium).
Fig. 16. Examples of Chroococcus. A, Ch. giganteus West; B, Ch. turgidus Naeg.; C and D, Ch. schizodermaticus West × 450 (after West).
Fig. 17. Gloeocapsa magma Kütz. × 450 (after West).
3. Gloeocapsa Kütz. (including Xanthocapsa). Globose cells with a lamellate gelatinous wall, forming colonies enclosed in a common sheath ([Fig. 17]); the inner integument is often coloured red or orange. These two genera form the gonidia in the family Pyrenopsidaceae. Gloeocapsa polydermatica Kütz. has been identified as a lichen gonidium.
Fam. nostocaceae. Filamentous algae unbranched and without base or apex.