95 



Wallroth has also made some observations on the growth and 

 duration of lichens. According to this investigator the homoeome- 

 rous lichens (Collemaceae and Parmeliaceae in part) are of quickest 

 growth and shortest duration ; some of them may begin growth 

 from a soredium and develop to maturity in the same year. He be- 

 lieved that the heteromerous lichens (Physciaceae, Parmeliaceae) 

 were endowed with almost eternal life. 



It must also be borne in mind that the growth of lichens is inter- 

 mittent. During warm moist seasons it is comparatively rapid ; 

 while during dry periods and extremely cold seasons growth is re- 

 duced to a minimum ; the limitation of the life-period must, there- 

 fore, be quite variable. 



One remarkable thing in the life-history of lichens is the readi- 

 ness with which these plants vanish before the progress of civiliza- 

 tion. Trees (large as well as small) growing in the city parks, 

 along much travelled roadsides, or in areas which have been under 

 cultivation for a long time, bear only a few of the lower forms (Le- 

 cidea, Graphis, Biatora, etc.) ; the higher types being almost en- 

 tirely wanting. This is evidently not due to mechanical interfer- 

 ence, but it is, perhaps, owing to the detrimental influence of the 

 fine dust which is everywhere stirred up in such situations. Rock- 

 lichens are the last to disappear. 



