21 G CLASS ASCOMYCETEAE 



As long ago as 1887 Alfred Moller reported that he had been able to grow 

 various, lichens in culture media in the absence of algae, and that these 

 produced small thalli, up to 1.5 cm. in diameter in some cases, with 

 typical cortex and medulla. In some tests the development of typical 

 spermogonia was observed. His experiments had to be cut short after four 

 months so that the further development of the still growing fungi could 

 not be followed. 



Vegetatively the Lecanorales vary from a loose branching mycelium 

 penetrating in all directions the gummy colony of a species of Nostoc, 

 the form of that colony setting the limits to the size and shape of the 

 lichen body {Collema), to a very complexly branched, firm thallus with 

 a cortical outer layer of hyphae enclosing the algal hosts and thus making 

 possible unlimited growth, without reference to the natural shape of the 

 unparasitized algal colony. 



The mycelium is in general slender, light-colored, septate, and branch- 

 ing. The septa are centrally pierced by a rather minute perforation. In 

 the forms with a cortex (the majority of lichens) the mycelial cells com- 

 posing it are short, broad, rather thick-walled and compacted together 

 into a pseudoparenchyma one to several cells in thickness. In the in- 

 terior of the lichen the mycelium is filamentous and loose. Usually the 

 algal hosts are found in definite layers. In some cases definite penetration 

 of the algal cell by the mycelium can be observed. Such cells are even- 

 tually killed. Geitler (1933) reported that more often no such penetration 

 can be seen but that the mycelium is applied to the host cell in the manner 

 of an appressorium. Just how the fungus draws its nourishment from the 

 alga in the absence of direct penetration is a matter of conjecture. Possibly 

 some substance secreted by the fungus increases the permeability of the 

 plasma membrane of the algal cells, thus permitting sugars and other 

 soluble food stuffs to diffuse out from the cell to be picked up by the 

 fungus. The fungus furnishes a certain amount of protection to the alga 

 and probably gives it a more equable habitat, protecting it from the rapid 

 extremes of drying and moisture, sun and shade, heat and cold. In so 

 far as this is true it is to the advantage of the alga. On the other hand, 

 however, the constant tribute levied in the form of food substances diffus- 

 ing out from the cells must reduce their vigor somewhat. It is worthy of 

 note that algae whose free-living development includes zoospore forma- 

 tion as a normal mode of reproduction usually have this entirely sup- 

 pressed, reproduction being limited mainly to fission. 



The earlier students of lichens consider the enclosed algal hosts to be 

 a part of the lichen organism, possibly reproductive in nature, hence the 

 term "gonidia" applied to them. By some they were considered to be 

 the photosynthetic organs of the lichen. Their similarity to algae was 

 early noted and Schwendener (18G7, 18G8) supplied the evidence that 



