THE NUTRITION OF PLANTS 
upon which it climbs, to the alga and fungus so intimately 
associated in a Lichen as to seem a single plant. In a nar- 
rower sense it includes only cases in which there is an inti- 
mate organic relation between the symbionts. This would 
include parasitism, the parasite and host being the sym- 
bionts, and the organic relation certainly being intimate. 
In a still narrower sense symbiosis includes only those cases 
in which the symbionts are mutually helpful. This fact, 
however, is very difficult to determine, and opinions vary 
widely as to the mutual advantage of the relation. How- 
ever large a set of phenomena may be included under the 
term symbiosis, we use it here in this narrowest sense, which 
is often distinguished as mutualism. 
(1) Lichens. The main facts ,of symbiosis in connec- 
tion with Lichens are presented in 194. That the fungus- 
symbiont can not live without the alga has been demon- 
strated, but whether the alga-symbiont derives any benefit 
from this association is a question in dispute. The latter 
can live independently of the former, but enmeshed by the 
fungus the alga seems to thrive and to live in situations 
which would be impossible to it without the protection and 
moisture supplied by the fungus-thallus. Those who lay 
stress on the first fact regard the Lichen merely as a pecul- 
iar case of parasitism, which has been called lielotism, or a 
condition of slavery, indicating that the alga is enslaved 
and even cared for by the fungus for its own use. Those 
who see an advantage to the alga in this association regard 
a Lichen as an example of mutualism. 
It may be of interest to know that artificial Lichens have 
been formed, not only by cultivating together spores of a 
Lichen-fungus and some Lichen-alga, but also by using 
" wild " Algae that is, Algae which are in the habit of living 
independently. 
(2) MycorrJiiza. The name means "root-fungus," and 
refers to an association which exists between certain Fungi 
of the soil and roots of higher plants, such as orchids, heaths, 
