I] SYMBIOSIS 19 



impeded and characteristic short, coral-like branches are formed. Sarcodes 

 sanguinea and Monotropa Hypopitys are non-green holosaprophytes with 

 exotrophic mycorhiza; in the former the whole root system is covered by 

 the mycelium of the fungus, in the latter the apices alone are free. 



In other cases infection seldom takes place in all roots; it is extensive 

 in soils rich in vegetable debris, and is absent in poor soils free from humus; 

 under these circumstances root hairs are formed and the roots function in 

 the normal way. When present it would appear that the mycorhiza not only 

 absorbs water and dissolved salts, but the vascular plant is further enabled 

 by its means to utilize directly the organic remains in the soil. 



The relation between an exotrophic fungus and its host, at least when 

 the latter contains chlorophyll, would appear to be much more casual than 

 is the case with endotrophic forms; like the latter it may arise as an at- 

 tempted parasitism on the part of the fungus, controlled and utilized by the 

 vascular plant. The absence of exotrophic mycorhiza in poor soils may depend 

 at least as much on the absence of saprophytic mycelia capable of causing 

 infection, as on the fact that a fungal associate would, under the circumstances, 

 be of little value to the green plant. Exotfophism would seem to be more 

 to the advantage of the fungal partner than endotrophism, since fructifica- 

 tions are often found on mycelia in exotrophic association with the roots of 

 vascular plants ; and some subterranean species, such as the truffles, fruit 

 only in the neighbourhood of appropriate trees. 



The fungi concerned in these curious relationships include representatives 

 of all the great groups; the endotrophic mycelium of the prothalli oi Lyco- 

 podiiim has been referred to the genus Pytkiuni, and that of the Marattiaceae 

 to Stigeosporium, a genus nearly related to Phytopkthora, that of several 

 orchids to Rhizoctonia {= Hypochmcs}) and of others to Nectria; the myce- 

 lium of species of Elaphomyces forms mycorhiza with the roots of Pinus and 

 other conifers; that of the Boleti with conifers and grasses and with willow, 

 poplar, hornbeam and birch, that of Tricholoma terreum with beeches and firs, 

 that of Lactarius piperatus and of species of Cortinarius with beeches and 

 oaks, that of species of Geaster with conifers. Reference has already been 

 made to the association of Armillaria mellea with the orchid Gastrodea 

 elata, and of Phoma-X-C&e. species with the Ericaceae. 



Finally there is little doubt that several of the Tuberaceae and Hymeno- 

 gasteraceae are frequent constituents of mycorhiza. 



PARASITISM AND SYMBIOSIS: BIBLIOGRAPHY 



1863 DE Bary, A. Recherches sur le ddveloppement de quelques champignons parasites. 



Ann. Sci. Nat. xx, p. 95. 

 1888 Ward, H. Marshall. Some Recent Publications bearing on the Sources of 



Nitrogen in Plants. Ann. Bot. i, p. 325. 

 1890 Oliver, F. W. On Sarcodes sanguinea^ Torr. Ann. Bot. iv, p. 303. 



