32 CONSTITUENTS OF THE LICHEN THALLUS 



parasitic condition. He proposed the happily descriptive designation of a 

 Symbiosis or conjoint life which was mostly though not always, nor in equal 

 degree, beneficial to each of the partners or symbionts. 



b. DIFFERENT FORMS OF ASSOCIATION. The type of association be- 

 tween the two symbionts varies in different lichens. Bornet 1 , in describing 

 the development of the thallus in certain members of the Collemaceae, 

 found that though as a rule the two elements of the thallus, as in some 

 species of Collema itself, persisted intact side by side, there was in other 

 members of the genus an occasional parasitism: short branches from the 

 main hyphae applied themselves by their tips to some cell of the Nostoc 

 chain (Fig. 9). The cell thus seized upon began to increase in size, and the 



Fig. 9. Physma chalazanum Am. Cells of Nostoc chains penetrated 

 and enlarged by hyphae x 950 (after Bornet). 



plasma became granular and gathered at the side furthest away from the 

 point of attachment. Finally the cbntents were used up, and nothing was 

 left but an empty membrane adhering to the fungus hypha. In another 

 species the hypha penetrated the cell. These instances of parasitism are 

 most readily seen towards the edge of the thallus where growth is more 

 active; towards the centre the attached cells have become absorbed, and 

 only the shortened broken chains attest their disappearance. The other 

 cells of the chains remain uninjured. 



In Synalissa, a small shrubby gelatinous genus, the hypha, as described 

 by Bornet and by Hedlund 2 , pierces the outer wall of the gelatinous alga 

 (Gloeocapsa) and swells inside to a somewhat globose haustorium which 

 rests in a depression of the plasma (Fig. 10). The alga, though evidently 



1 Bornet 1873. 2 Hedlund 1892. 



