LICHENS 



115 



two distinct kinds of plants, yet they exhibit a high degree of unity 

 and in many respects behave as single organisms. Perhaps the most 

 striking indication of a high degree of unity is seen in the bodies 

 called soredia. A soredium is a vegetative reproductive body con- 

 sisting of a small amount of fungus tissue enclosing a few algal cells. 

 It is the commonest and most efficient method of reproduction that a 

 lichen has. The lichen fungi, of course, produce spores but these 

 spores can never develop into lichens unless they chance to fall 

 among algse of a suitable species. The efficiency of the soredium, 

 therefore, is due to the fact that it keeps the two symbionts together. 



Fig. 55.— Lichens uu ruck. (Photograph by Bruce Fink.) 



It is practically the only case known in which two symbionts have a 

 common reproductive body. 



Another feature that makes lichens seem like unit organisms is 

 the fact that they are able to grow in extremely dry situations. 

 Fungi, as a group, are characteristically plants of medium conditions 

 with respect to water supply', while algffi as a group, are character- 

 istically plants of very wet situations, many of them growing in 

 water. But, when fungi and algae live together in the intimate 

 symbiotic relationship under discussion, the resulting organisms are 

 more resistant to dry conditions than any other group of plants. It 

 is for this reason that lichens form the outposts of the plant world 



