No. 400. ] FUNGUS AND ALGA IN LICHENS. 253 
than when the hypha merely enclose the gonidia, but the 
hyphz are parasitic in both cases. 
The foregoing argument, based on observations reported in 
detail elsewhere,! shows, I believe, that there are no physiologi- 
cal or structural reasons for supposing that the association of 
fungus and alga in lichens is other than the actual parasitism 
of the former upon the latter. If the lichens are merely fungi 
and algz associated together as parasite and host, these associa- 
tions are no less remarkable and interesting, though less moral, 
than if they were ideal alliances for mutual aid. Experiment 
has still to show whether the characters of the lichen thallus are 
formed by the fungus mainly, or by the fungus under the influ- 
ence of its host, how far substratum, illumination, nutrition, 
etc., affect the character of the association. Experimentation 
on lichens demands endless patience and considerable skill, for 
lichens grow with prodigious slowness, and the chances of a 
culture becoming infested with mould or bacteria before it has 
accomplished the purpose designed are only too great. Some 
of the problems puzzling systematic lichenologists can be 
solved. only when illuminated by experimental culture of the 
fungus component of the lichen thallus under definitely known 
conditions. 
STANFORD UNIVERSITY, CALIFORNIA, 
January, 1900. 
1 Peirce, G. J. Loc. cit. 
