52 



THE PLANT WORLD 



position on the face of a ledge, for example, may be such 

 as to allow the shredding out of the thallus into vertical 

 rugae, which at once changes the whole character of its 

 appearance. This condition is helped, no doubt, by water 

 running downward over its surface. 



"There is a low mist in the woods. It is a good day to 

 study lichens" .... "The lichens look rather bright 

 today .... when they are bright and expanded is it 

 not a sign of a thaw or a rain?" The majority of lichens are 



FIGURE 5. — A large rock specimen, showing loss of central 

 portions and peripheral ring. Note on the left a sloughed 

 portion showing black under side of thallus. 



much changed by the condition of the weather — a dry or rainy 

 day works a great change. The present species, however, is 

 only slightly brightened by wet weather. This is no doubt 

 explained by the fact that the green of the chlorophyll is al- 

 ways evident in this lichen's color, whereas in many it is only 

 during rainy weather that the alga symbiont is evident. 



