94 Minnesota Plant Life. 



for the most part, of developing their special types of fruit- 

 bodies. In addition to the algal propagation which goes on 

 among the algae of the lichen-body, and the fungal spore-forma- 

 tion which characterizes the fungal member of the firm, there 

 is commonly a production, superficially, of little granules, con- 

 sisting of two or three algal cells with a web of fungal threads 

 around them. Such minute granules, which may form a green- 

 ish dust over the surface of the lichen, are separated from the 

 region where they arise and serve as special partnership propa- 

 gative bodies. 



By means of the algal growth which goes on in the lichen 

 body, the algal partner increases in size wdiile it is protected by 

 its fungus neighbor. The fungus, too, grows, keeping pace 

 with the alga, and in this w^ay the double organism increases in 

 size. When the fungus bears fruits — a process which in some 

 lichen plant-bodies occurs very seldom, and in a few forms ap- 

 parently ne\"er- — the spores that are produced are in many in- 

 stances thought to be incapable of germination, and it would 

 appear that then the fungal partner has entirely lost the power 

 of existing independently of its alga. If the algae are removed 

 by careful methods from the plant-body they are usually able 

 to develop indc])endently and could not under such circum- 

 stances be distinguished from the same kinds of algae living 

 their ordinary life in pools or moist places. There are, how- 

 ever, exceptions to this rule, and some algae when removed 

 from their long accustomed partnership do not find it easy to 

 continue an independent existence, though in no case are they 

 so helpless as may be the fungi. It must doubtless be as- 

 sumed that lichens began to exist by the attachment of certain 

 fungi to algae, and that such a relation proved mutually bene- 

 ficial. Under it the algae were protected from desiccation by the 

 presence of the moist fungus threads and the fungi were able to 

 absorb nourishment from the algae, which they in turn, by 

 means of their leaf-green, were able to manufacture from gas 

 and water. .\s time went on these partnership structures began 

 to imi)r(jvc along i)aths favorable to the more perfect work of 

 the partncrshi]) as a whole, and hence in lichens there have 

 arisen leaf-like bodies, and even little tree-like stems with leafy 

 expansions u])oii tliem — not at all an unreasonable course of 



