44 



closest union in lichens whose algal symbiont is represented by Cys- 

 toccocus humicola Nag ; this alga occurs in far the greater number of 

 species. 



The forms of union between hyphae and algae are separated into 

 three kinds : i, Simple contact; 2, Extracellular haustoria; 3, 

 Intracellular haustoria. 



1. In this form of association the hyphae and algae are simply 

 contiguous. The hyphae seem to undergo no change in form or 

 development. This is especially the case in those lichens in which 

 JVostoc or Rivularia occurs (so-called gelatinous lichens) . In Lichina 

 for instance, the chains of Rivularia lie parallel and in contact with 

 the hyphal filaments. I have been unable to detect any influence 

 that the algae may have upon the direction of growth in the 

 hyphae. The same may be said of the Collemaceae. It is, never- 

 theless, evident from the life histories of the individuals of the groups 

 mentioned, that the symbiotic relationship is very highly developed. 

 From observation it also becomes evident that this contact-association 

 prevails in lichens bearing algae which are naturally enclosed by a 

 gelatinous coat ; what influence this coat has on the symbiotic asso- 

 ciation has not been explained. (PL i.f. 2.) 



2. In the majority of lichens the algae are enclosed by numerous 

 short terminal branches of the hyphal filament. Whenever a hyphal 

 filament comes in contact with an alga the contact surface is retarded 

 in growth, and, as a result, the filament begins to curve about the 

 alga. The presence of the alga also causes the development of short 

 hyphal branches which wind about the alga in the manner just men- 

 tioned. These branches are not only short, but they are usually 

 much thinner and their walls more delicate than the normal branches ; 

 they are also much more frequently septate. As the term extracel- 

 lular haustoria indicates, these enveloping terminal filaments never 

 penetrate the algal cell. The interchange of food substances takes 

 place by osmotic action through the hyphal and algal cell-walls. 

 The haustoria differ greatly in the degree of development. In some 

 instances they are few in number and very loosely united with the 

 algae ; when most completely developed they form a network or 

 covering closely united with the alga, so that it is almost impossible 

 to separate the symbionts. (PL i.f. j, 4.) 



3. This is, doubtless, the highest form of symbiotic association oc- 

 curring in lichens. Here we find that the haustoria of the hyphal 



