Minnesota Plant Life. 



93 



attached, and such kinds may often, too, be found upon tree- 

 trunks, fence-rails, twigs or the roofs of houses. A true rock- 

 lichen does not commonly occur upon wood, so that different 

 series of forms will be found if one examines the different hab- 

 itats where these plants are wont to display themselves. 



Structure of lichens. Lichens are very extraordinary plants, 

 or rather pairs of plants, for a 

 lichen is essentially a partner- 

 ship between a fungus and an 

 alga. Several different groups 

 of algae are employed in the 

 building up of lichen bodies, 

 especially the blue-green algae 

 such as the water-flower 

 and the bright-green algae. I 

 do not know of any lichens 

 which employ red or brown al- 

 gae in their partnership. Sev- 

 eral varieties of blue-green and 

 bright-green algae are con- 

 cerned, but a particular species 

 of lichen rarely exhibits more 

 than one kind of alga and one 

 kind of fungus in its partner- 

 ship-structure. As will be ex- 

 plained, such partnerships are 

 self-perpetuating, and the part- 

 nership comes to have the ap- 

 pearance and very much the 

 character of a plant-unit, so 

 much so, indeed, that for con- 

 venience lichens are generally 

 viewed as independent unit- 

 plants rather than as the 

 double organisms which in reality they are. 



If one makes a very thin slice through the plant-body of a 

 lichen it will be found to consist of certain algal cells or fila- 

 ments quite able to propagate after their kind, and these en- 

 closed in a tangle of fungus filaments which are equally capable, 





FIG. 36. "Old man's teard." A lichen 

 growing attached to the twigs of tam- 

 arack. I<ake Superior, north shore. 

 Natural size, six inches in length. After 

 photograph by Professor Bruce Fink. 



