174 MINNESOTA BOTANICAL STUDIES. 



the same sort of development on the upper surface as in the 

 trichomatic hyphas of some Peltigeras. There is more or less 

 of the same development of such structures in other genera as 

 in Physcia ciliaris (L.) DC. and P. hisptda (Schreb.) Tuck, and 

 even in some Cladonias, though usually quite overlooked in the 

 last. These structures may be protective in part, are surely re- 

 lated to some extent to moisture .conditions, but they seem to be 

 better developed as a whole toward the north and no doubt have 

 a temperature relation as well. Thus when we add the many 

 adaptations in thallus structure, which have been dwelt on in 

 the last paper of this series and will receive attention in the fol- 

 lowing pages, and which are surely more than purely mechan- 

 ical adaptations, it becomes very apparent that to dismiss lich- 

 ens ecologically, as is sometimes done, with the mere statement 

 that they can endure complete desiccation and therefore have 

 and need no special adaptations is quite erroneous. That they 

 can endure great desiccation is to be admitted ; but since they 

 have the unusual power of absorbing moisture directly from the 

 atmosphere, it is doubtful whether the drying process often 

 . goes so far as has been commonly supposed. However much 

 of drying the lichens may be able to endure, we are con- 

 vinced that the condition is unfavorable to them and that, like 

 xerophytes in general, they show certain structural adaptations, 

 .and these we shall consider as well as may be done in a gen- 

 eral survey. Moreover, in this survey of a large region in- 

 volving many conditions, a large number of geological, climatic, 

 physiographic, hydrodynamic, biotic, general atmospheric and 

 -even some of the more important edaphic factors could not re- 

 -ceive the attention deserved. 



Beginning with the previously studied and better known for- 

 mations and passing to the less known and finally to a few here 

 introduced for the first time, we may take first the following : 



LECANORA FORMATION OF EXPOSED (USUALLY HORIZONTAL) 

 ROCKS. (RAINY LAKE CITY.) 



Parmelia saxatilis (L.) FR. var. panniformis ACH. 



Parmelia olivacea (L.) ACH. var. prolixa ACH. 



Parmelia conspersa (EHRH.) ACH. 



Physcia stellaris (L.) TUCK. var. apiola NYL. 



Physcia cassia (HOFFM.) NYL. 



Placodium elegans (LINK.) DC. 



