1893.] Symbiosis and Mutualism. 511 
Fortunately, the lichens exhibit several intermediate forms, 
and enable us to see the relation between the phenomenon 
found in the commoner lichens and ordinary parasitism. 
Lichenologists have, for a long time, distinguished, under 
one name or another, two classes of lichens. In the one group 
the thallus is entirely or substantially homogeneous—there is 
no differentiation into rind, medulla, etc. In the other there 
there is a well-defined rind, and gonidial and other zones are 
differentiated. The former have been called homcomerous 
lichens, the latter heteromerous lichens. In the first group 
the alga is the principal part of the lichen. The hyphe grow 
within the mass of algal cells and follow them in their growth. 
To this class belong Collema and like genera, which are fungi 
parasitic upon Nostoc, Scytonema, etc., and growing within the 
gelatinous membranes and sheaths enveloping those alge. 
Here there is symbiosis—a living together of parasite and host 
—but no one will contend that there is mutualism. 
In the second group the fungus is the principal part of the 
lichen. It contains in its thallus a zone of alge, but they fol- 
low the growth of the thallus, and their bulk is a small pro- 
portion of the whole lichen. In these lichens the alge are 
Protococcoidex or Palmellacex, ete., and to the different mode of 
growth of these alge the difference is largely to be attributed. 
Between these groups there are a number of forms, usually 
classed as heteromerous lichens, which, nevertheless, show no 
differentiation of medulla and rind, and in which the thallus 
consists of a weft of slender hyphe growing around filaments 
of Chroolepus and like forms. Still another fact is important 
in this connection. Some of the genera of this intermediate 
group have species which contain no gonidia and are sapro- 
phytes upon bark, and indeed the parasitic species are often 
saprophytes during a part of their existence. Many genera of 
fungi exhibit the same phenomenon. 
It is seen, then, that mutualism does not exist in all lichens, 
and that the steps from an ordinary case of parasitism, such as 
that exhibited by the homceomerous lichens, which consist of 
a mass of algal cells permeated by the hyphe of a fungus and 
often distorted by it, to the peculiar case of the heteromerous 
