THE FOOD OF PLANTS 87 



upon which it climbs, to the alga and fungus GO 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 r ider 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 connection 

 with Lichens were presented in 53. 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 heloti*m, 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) Mycorrhiza. 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, 



