THE NUTRITION OF PLANTS 159 



upon which it climbs, to the alga and fungus so intimately 

 associated in a Lichen as to seem a single plant. In a 

 narrower sense it includes only cases in which there is an 

 intimate organic relation between the symbionts. This 

 would include parasitism, the parasite and host being the 

 symbionts, and the organic relation certainly being inti- 

 mate. 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 often vary widely as to the mutual advantage in 

 certain cases. However 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 

 fungus 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) Mycorhiza. — 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 higher 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 fila- 

 mentous fungi.'' It is clearly of advantage to roots to 

 relate themselves 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 competition for the nutrient materials of the forest 

 soil. It is doubtful whether many vascular green plants 



