54 GUIDE TO THE STUDY OP LICHENS. 



ing species. The same thing occurs among foliose 

 and fruticose lichens. A great many lichens show a 

 decided preference for mosses, over which they spread 

 in considerable profusion, finally killing the mosses, 

 not so much as the result of drawing any nourishment 

 from them, but rather as the result of mechanical in- 

 terference and the shutting off of air and sunlight. 



In general it may be stated, that lichens occur plen- 

 tifully from the northern limit indicated to the tropics. 

 Some lichens are essentially southern, others northern 

 or western. Only a very few are typically eastern. 



It is quite important that the collector should have 

 some knowledge of the vertical distribution of lichens ; 

 this will avoid much waste of time in searching for 

 lichens in places where they do not occur. For exam- 

 ple, Gyrophoras and Umhilicarias do not occur at 

 slight elevations ; in fact, the lowlands yield only com- 

 paratively few lichens. As a rule, lichens which occur 

 at low levels in the arctic regions also occur at high 

 altitudes in the warmer or tropical regions. 



As already indicated, mountainous timber regions 

 yield the most numerous and finest specimens of 

 lichens ; prairies and open lowlands yield the poorest 

 and fewest. It also seems that there is a narrow belt 

 in the immediate vicinity of the coast which is deficient 

 in lichens ; this is perhaps due to excessive moisture, 

 which is detrimental to their existence. We have 

 elsewhere indicated that lichens flee from civilization. 

 The reasons for this are not definitely known ; it is 

 very likely due to the dust and smoke (impure atmo- 

 sphere) of the cities ; they require pure air for their 



