LICHEN GONIDIA 39 



Sutherland ' in an account of marine microfungi. One of these, a species of 

 Mycosphaerella, was found on Pelvetia canaliculata, and Sutherland claims 

 that as no apparent injury was done to the alga, it was a case of 

 symbiosis and that there was formed a new type of lichen. The mycelium, 

 always intercellular, pervaded the whole host-plant, and the fungal fruits 

 were invariably formed on the algal receptacles close to the oogonia. Their 

 position there is, of course, due to the greater food supply at that region. 

 Both fungus and alga fruited freely. A closer analogy could have been found 

 by the writer in the smut fungus which grows with the host-cereal until 

 fruiting time; or with the mycorrhiza of Calluna which also pervades every 

 part of the host-plant without causing any injury. In the true lichen, the 

 alga, though constituting an important part of the vegetative body, takes no 

 part in reproduction, except by division and increase of the vegetative cells 

 within the thallus. The fruiting bodies are always of a modified fungal 

 nature. 



2. PHYSIOLOGY OF THE SYMBIONTS 



The occurrence of isolated cases of parasitism — the fungus preying on 

 the alga — in any case leaves the general problem unsolved. The whole 

 question turns on the physiological activity and requirements of the two 

 component elements of the thallus. From what sources do they each 

 procure the materials essential to them as living organisms? It is chiefly 

 a question of nutrition. 



A. Nutrition of Algae 



a. Character of Algal cells. Gonidia are chlorophyll-containing 

 bodies and assimilate carbon-dioxide from the atmosphere by photo- 

 synthesis as do the chlorophyll cells of other plants. They also require 

 water and mineral salts which, in a free condition, they absorb from their 

 immediate surroundings, but which, in the lichen thallus, they must obtain 

 from the fungal hyphae. If the nutriment supplied to them in their inclosed 

 position be greater or even equal to what the cells could procure as free- 

 living algae, then they undoubtedly gain rather than lose by their asso- 

 ciation with the fungus, and are not to be considered merely as victims of 

 parasitism. 



b. Supply of Nitrogen. Important contributions on the subject of 

 algal nutrition have been made by Beyerinck^ and Artari'. The former 

 conducted a series of culture experiments with green algae, including the 

 gonidia of Physcia {Xanthorid) parietina. He successfully isolated the 

 lichen gonidia and, at first, attempted to grow them on gelatine with an 

 infusion of the Elm bark from which he had taken the lichen. Growth was 



' Sutherland 1915. '^ Beyerinck 1890. ' Artari 1902. 



