462 IVAN E. WALLIN 
In a discussion of algae and their relationships, Campbell 
(99) refers to some chlorophyl-bearing algae that have developed 
a symbiotic relationship to some higher plants. There is no 
reason to suppose that all unicellular green or blue-green algae 
belong to a single species. ‘To the contrary, botanists recognize 
many species. The fact that some species of algae may be 
recognized in a symbiotic relationship with higher plants indicates 
the possibility that other species may have developed an absolute 
symbiosis with some higher plants. After an organism has de- 
veloped absolute symbiosis, it is not conceivable that it is capable 
of an independent existence under natural conditions. It is, 
also, to be expected that an absolute symbiont would lose some 
characters and properties that the free-living progenitor possessed. 
One of the best known and oldest examples of symbiosis is 
furnished by the lichens. This symbiosis was first described by 
Schwendenen in 1860, and has since been confirmed by the in- 
vestigations of a host of authors. The lichens furnish not only 
an indisputable example of absolute symbiosis, but also varying 
degrees of symbiotic relationships. 
In the lower lichens the algal symbiont is capable of independ- 
ent existence. While it has not been demonstrated, Schneider 
(97) and other investigators believe that the fungal symbiont 
in some lowly lichen may also be capable of independent exist- 
ence. In the most highly developed lichens the symbiosis is 
complete or absolute; neither symbiont can live without the 
other. Here we have an example of symbiosis that, apparently, 
is identical with every detail in the relationship of chloroplast to 
host plant. : 
Between the most lowly and the most highly developed lichens, 
various degrees of symbiosis are represented. This gradation of 
symbiosis is accompanied by differences in reproduction. In the 
lowly lichens, where symbiosis is incomplete, the algal and fungal 
symbionts reproduce independently. In higher forms, but where 
the symbiosis is still incomplete, an accessory reproduction may 
take place by a type of budding in which both algal and fungal 
symbionts are represented. Inthe most highly developed lichens, 
where there is absolute symbiosis, both the algal and fungal 
