197 
therefore be considered as an independent group. Based upon 
morphological and physiological considerations this group would 
naturally belong midway between fungi and algae. 
In conclusion I shall offer a few suggestions on the probable 
origin and phylogeny of lichens. There is little doubt that vari- 
ous subdivisions of lichens indicate a polyphyletic origin. Of 
this polyphylogeny either one or all of several forms may have 
- occurred. For example, in ascolichenes, a certain fungal type may | 
have (during its phylogenyas a lichen symbiont) become so modi- 
fied by its symbiosis with a given algal type, as to enable it to as- 
sociate with other algal species; or it may be that the same algal 
type became adapted to one or even several fungal types. As to 
what the conditions actually were we are at present scarcely able 
to say. Of one thing we are, however, certain and that is that a 
lichen is the result of the mutualistic association of a fungal and 
an algal type. Though in general I agree with Reinke as to the 
origin of lichens, yet I am not so ready to assume (theoretically) 
that Co//ema represents the oldest lichen type. Cod/ema is the re- 
sult of the symbiotic association of the alga Mostoc with some 
fungus whose ancestral type is not definitely known. The mass 
of the lichen structure consists of the alga. As compared with 
some other lichens the following are some of the reasons why 
Collema does not represent the lichen prototype: I. The alga 
has undergone considerable change by way of adapting itself to 
new environments. Originally it was no doubt accustomed to a 
high degree of moisture (as is its present homologue, Nostoc com- 
mune), while in its present form as a lichen it is able to exist on 
tree trunks, rocks, etc., as most other lichens. 2. Its thallus 
shows a considerable degree of differentiation, as exemplified in 
the closely allied genera Hydrothyrio, Polychidium, Leptogium and 
Mallotium. 3. Spores have probably become considerably degen- 
erated as indicated by their thin colorless walls, and in many 
Cases indistinct septae. As a rule apothecia are few, though there 
are exceptions to this. 4. Soredia, though not numerous, are more 
frequently present than in many other lichen forms. 
The above are the main reasons why Collema is perhaps not 
the prototype of lichens. In my opinion the true prototype of 
oe lichens is perhaps to be found in those structures which were for- 
