INTRODUCTION 19 



It would be helpful if definite statements could be made about the kinds of 

 rocky substrata. However, some lichens grow on several different kinds of rocks, 

 and collectors often do not state the kind of rock on which a lichen is found. 



It would also be helpful if more could be given about the habitat, but state- 

 ments are usually fragmentary, and the facts desired could be obtained only by- 

 extensive studies in many localities. 



Lichens are not commonly confined to a particular species of tree, and the 

 statements regarding their occurrence on certain trees may indicate that there 

 has been little observation rather than that the lichen in question is not found 

 on other trees. 



For many crustose lichens a very limited distribution is given. It is highly 

 probable that careful and critical collecting in many regions would greatly extend 

 the known range of what now appear to be rather rare forms. 



