54 



POPULAR HISTORY OF LICHENS. 



phoses ; in doing so, we shall be led incidentally to consider 

 the chief circumstances which promote or retard the growth 

 of the th alius. The study of these causes and circumstances 

 is one of great importance to the young lichenologist, as a 

 key to the phases or varieties of species. In their influence 

 on the Lichen-thallus no agents are more powerful than 

 moisture, light, and heat ; hence we find Lichens at- 

 taining their maximum development under maximum con- 

 ditions of moisture, light, and temperature, that is, in the 

 the tropics. At the same time, however paradoxical it may 

 appear, no plants are more independent of the influence of 

 these agents, for they vegetate in the driest and coldest 

 regions yet discovered by man. Their vitality is more per- 

 sistent than that of any higher plants ; it is frequently sus- 

 pended for long periods by drought, but growth is at once 

 stimulated by the slightest moisture, which is greedily ab- 

 sorbed by all points of the surface. The effect of moisture 

 in producing a change of colour has been mentioned ; it is 

 sometimes more remarkably productive of change in con- 

 sistence. The flaccid membraniform Collema, when mois- 

 tened, swells up into a jelly-like mass, resembling many of 

 the Alga. Some authors indeed believe that Lichens might 

 be caused to assume algoid characters by making them ve- 



