246 THE AMERICAN NATURALIST. [Vor. XXXIV. 
nous discussion of symbiosis in lichens. In spite of all the evi- 
dence, old and new, and in spite of common sense, the lichens 
are only too often described as furnishing the most perfect 
examples of the mutually beneficial association of two utterly 
different kinds of organisms, — one of these able to lead a normal 
and perfect existence when by itself, the other absolutely de- 
pendent for food upon other organisms, living or dead. Let 
me, therefore, once more direct the attention of biologists to a 
scrutiny of these peculiar forms. 
Every one now admits that lichens consist of two distinct and 
separable parts, of colorless fungus-like hyphze, which form the 
greater part of the mass of the lichen, and of chlorophyll-con- 
taining alga-like cells, known as gonidia. Repeatedly it has 
been shown that the gonidia of lichens are not merely alga-like, 
but are alge, often found free in nature, growing not only on 
‘the same surfaces as furnish the seat of attachment of lichens, 
but on many others also, growing and multiplying as healthy 
individuals whenever external conditions make active life pos- 
sible, passing over other periods in one or another of the rest- 
ing conditions known to biologists. No one now questions 
that lichen gonidia are alge, and that with due care gonidia 
can be identified with already known species of alga found 
outside of lichen associations. Identification on mere inspec- 
tion of the gonidia in the lichen is not always possible, nor 
would it always be correct. For example, Hedlund? says that 
the alga which forms the gonidia in species of Micaria usually 
looks like Glceocapsa and frequently occurs on rotten wood, 
but that, when cultivated on this wood under abundant illumi- 
nation and free from fungus enemies or algal competitors, the 
alga multiplies rapidly, becomes deep green, the daughter-cells 
separate, and none has the thick gelatinized wall which is one 
of the characters of Gloeocapsa. Conditions wholly external 
to the algal cells, especially such influences as affect their free- 
dom, cause these cells to depart from their Protococcus type 
and to develop qualities and habits characteristic of a wholly 
1 Hedlund, T. Kritische Bemerkungen über einige Arten der Flechtengat- 
tungen Lecanora, Lecidea, und Micarea. Bikang till K. Svenska Vet. Akad. 
Handlingar, Bd. xviii. Stockholm, 1892. 
2 
