WHAT ARE LICHENS? 29 



fore, in mind, that in the discussion of lichens we are 

 concerned with certain genera of the lower algae be- 

 longing to the Chlorophyceae and the Cyonophyceae, and 

 a division of the higher fungi known as sac-fungi 

 (Ascomycetes), we shall now briefly ex^Jain the phylo- 

 genetic history of a lichen. The origin of one lichen is 

 in all essential respects similar to that of any other, so 

 that one example will suffice to make clear the origin 

 of all lichens. 



I. THE ORIGIN OF A LICHEN. 



As to the time when the first lichen was formed 

 nothing definite is known. We may, however, safely 

 conclude that they existed in the geological periods 

 along with algae and fungi. We are certain that they 

 could not have existed before the development of fungi, 

 as explained above ; we also know from what has gone 

 before that algae antedate fungi, from which we con- 

 clude that lichens began their existence many ages 

 later than the first alga? and fungi. From the great 

 structural and functional specializations that lichens 

 have undergone, we must again conclude that they be- 

 gan their existence far back in the geologic ages, that 

 is, millions of years ago. 



If we select one of the foliose Parmelias as a type, 

 the hypothetical assumption is that its origin was as 

 follows : A sac-fungus (Ascomycete) evidently belong- 

 ing to the genus Patella, found it diflicult to maintain 

 a thrifty existence as a saprophyte ; the organic food- 

 supply was no longer adequate to keep up sufiicient 

 energy to maintain a successful struggle for existence. 



