H 



VI. PERIOD. 



From Schwendener (1868) to Reinke (1894). 



This is by far the most important period in the entire history of 

 lichenology. It is marked by the recognition of the true nature of 

 lichens and their classification as modified fungi. Since most of the 

 literature of this period is readily accessible I shall not review it at 

 length and shall limit myself to a very brief outline of the work 

 done during this period, mentioning only a few of the leading in- 

 vestigators. 



The most important work of the period was the discovery of 

 the dual nature of lichens. That is, a lichen consists of a fungal 

 and an algal portion associated in an intimate, organic union. Al- 

 though Schwendener is generally credited with having made 

 known this discovery, it must not be forgotten that the preparatory 

 work was done in the preceding period ; also that Schwendener did 

 not at first believe in the dual nature of lichens. Not until the year 

 1868 (79) did he express the opinion that the gonidia of various 

 lichens corresponded to certain low forms of algae. His conclu- 

 sions of that time may be summarized as follows : 



1. There is no direct proof of any genetic relation between the 

 gonidia and the hyphal elements. 



2. The cell-walls of the gonidia have a different chemical be- 

 havior from the membranes of the hyphae ; the former react 

 similarly to those of algae, the latter similarly to those of fungi. 



3. As to structure and development the various forms of gonidia- 

 correspond to different forms of algae. The resemblance is so close 

 that in many cases a given isolated gonidium cannot be distinguished 

 from the corresponding alga. The algal types are as follows : 



(a) The majority of heteromerous lichens (Usnea, Bryofogon, 

 Evernia, Physica, Ana-ptychia, Imbricaria, Parmelia, etc.), con- 

 tain species of the algal genus Cystococcus Naeg. ( C. humicola and 

 related forms). 



(b) Some other heteromerous lichens contain species of Pleuro- 

 coccus Menegh. (P. vulgaris and related forms) . 



(c) In Rocella we find the algal genus Exococcus Naeg. 



(d) Omphalaria and other lichens with blue-green gonidia con- 

 tain various representatives of the Chroococcaceae as Gloeocafsa, 

 Ckroococcus, and perhaps other related forms. 



