THE FOOD OF PLANTS 87 



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 in certain cases. 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. — A lichen is a complex made up of a fun- 

 gus and an alga living together. It is certain that the fun- 

 gus cannot live without the alga, but the alga can live 

 without the fungus. Hence it seems plain that this rela- 

 tion is not one of mutual helpfulness, but that the fungus 

 is living upon the alga, as any other parasite lives upon its 

 host (see § 194). 



(2) MycorMza. — The name means "root-fungus," and 

 refers to an association which exists between certain Fungi 

 of the soil and roots of higher plants. It was formerly 

 thought that mycorhiza occurred only in connection with 

 a limited number of liigher plants, such as orchids, heaths, 

 oaks, etc., but more recent study indicates that probably 

 the large majority of vascular plants (that is, plants with 

 true roots) possess it, the water plants being excepted (Figs. 

 149, 150). It has been found that the humus soil of forests 

 is in large part " a living mass of innumerable filamentous 

 fungi." It is clearly of advantage to roots to relate them- 

 selves to this great network of filaments, which are already 

 in the best relations for absorption, and those plants which 

 are unable to do this are at a disadvantage in the competi- 

 tion for the nutrient materials of the forest soil. It is 

 doubtful whether many vascular green plants can absorb 



