52 



CONSTITUENTS OF THE LICHEN THALLUS 



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^ 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^: 



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 Kiitz. Globose or subglobose cells forming large 

 colonies surrounded by a common gelatinous layer (gonidia of Coris- 

 ciuni). 



3. Gloeocapsa Kiitz. (including Xanthocapsd). Globose cells with a 



Fig. 17. Gloeocapsa magma 

 Kiitz. X 450 (after West). 



Fig. 16. Examples of Chroococcus. A, Ch. giganteus 

 West; B, Ch. turgidus Naeg. ; C and D, Ch. schizo- 

 dermaticus West x 450 (after West). 



lamellate gelatinous wall, forming colonies enclosed in a common sheath 

 (Fig. 17); the inner integument is often coloured red or orange. These 



^ Nylander (1866) gave the term "gonimia" to the blue-green algae of the Phycolichens, retaining 

 the term " gonidia " for the bright-green algae of the Archilichens : the distinction is not now main- 

 tained. 



^ For further details see also the chapter on Classification. 



