LICHEN ALGAE 61 



only less evident but much more faintly coloured. In Synalissa, a minute 

 shrubby lichen which has the same algal constituent, the tissue of the thallus 

 is more highly evolved, and in it the red colour can barely be seen and 

 then only towards the outside; at the centre it disappears entirely. The 

 long chaplets of Nostoc cells persist almost unchanged in the thallus of the 

 Collemaceae, but in heteromerous genera such as Pannaria and Peltigera 

 they are broken up, or they are coiled together and packed into restricted 

 areas or zones. The altered alga has been frequently described as Polycoccus 

 punctiformis. A similar modification occurs in many cephalodia, so that the 

 true affinity of the alga, in most instances, can only be ascertained after free 

 cultivation. 



Bornet 1 has described in Coccocarpia molybdaea the change that the alga 

 Scytonema undergoes as the thallus develops : in very young fronds the 

 filaments of Scytonema are unchanged and are merely enclosed between 

 layers of hyphae. At a later stage, with increase of the thallus in thickness, 

 the algal filaments are broken up, their covering sheath disappears, and the 

 cells become rounded and isolated. Petractis (Gyalecta) exanthematica has 

 also a Scytonema as gonidium, and equally exact observations have been 

 made by Funfstiick 2 on the way it is transformed by symbiosis: with the 

 exception of a very thin superficial layer, the thallus is immersed in the 

 rock and is permeated by the alga to its lowest limits, 3 to 4 mm. below the 

 surface, Petractis being a homoiomerous lichen. The Scytonema trichomes 

 embedded in the rock become narrower, and the sheath, which in the 

 epilithic part of the thallus is 4/4 wide, disappears almost entirely. The 

 green colour of the cells fades and septation is less frequent and less regular. 

 The filaments in that condition are very like oil-hyphae and can only be 

 distinguished as algal by staining reagents such as alkanna. They never 

 seem to be in contact with the fungal elements : there is no visible appearance 

 of parasitism nor even of consortism. 



b. CHLOROPHYCEAE. As a rule the green-celled gonidium such as 

 Protococcus is not changed in form though the colour may be less vivid, but 

 in certain lichens there do occur modifications in its appearance. In Micarea 

 (Biatorind) prasina, Hedlund 3 noted that the gonidium was a minute alga 

 possessing a gelatinous sheath similar to that of a Gloeocapsa. He isolated 

 the alga, made artificial cultures and found that, in the altered conditions, 

 it gradually increased in size, threw off the gelatinous sheath and developed 

 into normal Protococcus cells, measuring 7 to lOyu, in diameter. The gelatinous 

 sheath was thus proved to be merely a biological variation, probably of 

 value to the lichen owing to its capacity to imbibe and retain moisture. 

 Zukal 4 also made cultures of this alga, but wrongly concluded it was a 

 Gloeocystis. 



1 Bornet 1873. 2 Fiinfstiick 1899. 3 Hedlund 1892. 4 Zukal 1895, p. 19. 



