76 GUIDE TO THE STUDY OF LICHENS. 



SECTION 11. 

 KEYS TO THE STUDY OF LICHENS. 



Keys to a system of plants are of value, but this 

 value is generally overestimated The usefulness of a 

 key depends largely upon the one who makes it ; if 

 he has a thorough knowledge of the subject, he is 

 generally able to construct a key which will be found 

 highly useful by others. Any key, whether natural or 

 artificial, should contain the fewest words possible 

 consistent with clearness ; close attention must also be 

 given to co-ordination and subordination. 



Two keys are here proposed : one, the artificial, in- 

 tended for those who cannot secure the use of a com- 

 pound microscope, and who are dependent upon the 

 eye and a pocket-lens to aid them ; the second, a nat- 

 ural key, recommended to those who have a compound 

 microscope at their disposal. 



It will be noticed that in the artificial key the thal- 

 lus and apothecia are of prime importance ; in the 

 natural key the algae and spores are of first importance 

 in the determination of genera. No attempt has been 

 made to arrange keys to the species. 



In the artificial key it was found necessary to com- 

 bine several genera in some instances, because no marked 

 macroscopic differential characters were noticeable. 

 Furthermore, several insignificant genera have been 

 wholly omitted. 



