1 89 1 -9 2.] On the Dissociation of a Lichen. 33 



VII.— ON TEE DISSOCIATION OF A LICHEN. 



By Mr WM. C. CRAWFORD, M.A. 

 (Read Feb. 24, 1S92.) 



A lichen, as is well known, consists of an alga and a fungus 

 living together for mutual advantage. The association of the 

 two organisms is so intimate, that lichens were considered till 

 about twenty-five years ago as simple plants forming by them- 

 selves a division of Cryptogams. 



The theory of the compound nature of lichens was first 

 published in 1868 by Professor Schwendener, who was led to 

 his theory by anatomical considerations alone. The theory 

 was strongly supported by philosophical botanists, while the 

 lichen collectors opposed it with a virulence remarkable in 

 scientific literature. The problem of the simple or compound 

 nature of lichens may be solved in two ways. The suitable 

 fungus, usually one of the discomycetes or pyrenomycetes, 

 may be grown from spores, and the cells of algae brought into 

 contact with it, and so a lichen may be manufactured. This 

 is the synthetic method. Or a complete lichen may be 

 separated into its constituent living elements, and an attempt 

 may be made to grow these apart from each other. This is 

 the analytic method. The chief interest of the problem does 

 not lie in its solution merely, but rather in new problems 

 which arise out of it. The synthetic method has been pur- 

 sued successfully by several distinguished botanists ; the 

 analytic method of approaching the problem — the dissociation 

 of lichens — has hardly been attempted at all. 



One of the first to try to build up a lichen was the Dutch- 

 man Treub (' Botanische Zeitung,' 1873). He planted lichen 

 spores in a drop of distilled water on microscope slides kept 

 in an atmosphere saturated with moisture. A little ash of 

 lichen was added as nutritive material, and after the spore 

 had germinated for a fortnight or so, gonidia were brought 

 into contact with the growing filament. The fungal filaments 

 attached themselves to the gelatinous envelope of the green 

 cells, and then grew on more vigorously than before, showing 



vol. in. c 



