Ill 



"Lichens," in the Cambridge University Botanical Series, and 

 the introduction to Bruce Fink's "Lichens of Minnesota," it 

 may be stated that after three centuries of varying ideas as to 

 the real nature of lichens, it is now generally accepted that they 

 are dual in nature, a symbiotic association of algae and fungi. 

 Symbiosis (living together) is found in other forms of plant life, 

 but nowhere else in such an extensive, intimate and efficient a 

 manner as in lichens. Schwendener, who first definitely pro- 

 pounded the idea of lichen symbiosis, in 1860, regarded the rela- 

 tion of the fungus as that of a parasite upon the alga, though 

 not a deadly one, and expressed this in the following picturesque 

 language. 



"All these plants are not individuals in the common sense of 

 that term; they are rather colonies, consisting of hundreds and 

 thousands of individuals, of which one holds the mastership, 

 while the others in eternal captivity prepare the nourishment 

 for themselves and their masters. The Master is a Fungus of the 

 Class of Ascomycetae, a parasite which lives on the labor of 

 others; its slaves are green Algae, which it seeks or lays hold of 

 and forces into its service. It encloses them, as a spider its prey, 

 with a network of delicate tissue, which is gradually trans- 

 formed into an impervious integument. But while the spider 

 sucks the life out of its prey, the Fungus stimulates the Algae 

 in its grasp to greater activity, to a more vigorous increase, and 

 thereby renders possible a luxuriant growth and promotes the 

 welfare of the whole colony." 



There has been much discussion since Schwendener's time 

 as to whether the association is parasitic or mutually bene- 

 ficial. Reinke suggested the term "Consortium" as expressing 

 his idea that the symbiosis was a state of mutual growth and 

 interdependence, in which the algal cells produced food products 

 from the substratum, or the air, enabling the fungal cells to 

 develop into their often beautiful and complicated fruiting struc- 

 tures, which produce spores that germinate only if they find 

 within a very brief time after dispersal free algal spores of 

 species similar to those which the parent lichen had in partner- 

 ship. 



That lichens renew themselves in this manner, as well as in 

 others to be referred to later, is proved by the fixity of genera 

 from very ancient geological periods (at least as old as the 



