_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_).
[Illustration: Fig. 16. Examples of _Chroococcus_. A, _Ch. giganteus_ West; B, _Ch. turgidus_ Naeg.; C and D, _Ch. schizodermaticus_ West × 450 (after West).]
[Illustration: 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.
NOSTOC Vauch. Composed of flexuous trichomes, with intercalary heterocysts (colourless cells) (Fig. 18). Dense gelatinous colonies of definite form are built up by cohesion. In some lichens the trichomes retain their chain-like appearance, in others they are more or less broken up and massed together, with disappearance of the gelatinous sheath (as in _Peltigera_); colour mostly dark blue-green.