It has been reported that the mycorrhiza of many herbaceous plants is of dual character, involving primary infection by a fungus with the characters just described followed by secondary infection by one of the type associated with orchid roots. Whether this reported "double infection" is of biological significance is at present unknown.
Role of Mycorrhiza in Plant Growth.—There can be little doubt that the mycorrhizal habit had its origin in parasitic attack on the part of certain soil fungi, or that existing mycorrhiza plants are the survivors of a long and perilous evolutionary history. Many must be regarded as cases of compromise fluctuating be tween an almost complete resistance to fungus invasion and a re markable toleration of the same. That there is an exchange of nutrient material in practically all cases can hardly be doubted but, in the absence of information based on the behaviour of individuals with and without mycorrhiza, attempts to express the shifting relationships in terms of mutual advantage or the reverse are premature and often misleading.
In certain groups of plants mycorrhiza has become a regular feature in the life history, a condition of delicately balanced equilibrium between fungus and host has been established and critical experimental study has placed certain facts beyond doubt. In both orchids and heaths the normal development of the seedling is bound up with the presence and activities of mycorrhizal fungi showing marked adaptation to individual species, accompanied in members of Ericaceae by a distribution of mycelium through out the tissues so extensive that it is difficult to conceive of the individual plant as other than a single entity of double constitu tion. In orchids, the traditional view attributes a beneficial role to the root fungi based on their utilization of organic soil con stituents and the wholesale digestion of mycelium that takes place subsequently in the root-cells. A claim has been advanced for fixation of atmospheric nitrogen by the root fungi of several orchid species.
It may be safely concluded that mycorrhiza possesses a vital significance for trees and other plants growing in woodland soils, and there is reason to believe that the reaction and constitution of the soil are important factors in promoting its healthy develop ment and functioning. In the Ericaceae, Calluna and many of its
allies are characteristic and abundant members of the vegetation of humus soils thus pointing to similar conclusions, while in this group there is likewise evidence that the capacity to draw in directly upon the organic compounds in the soil humus is sup plemented by the ability of the root fungi to utilize atmospheric nitrogen. In both orchids and heaths the obligate character of the association is probably confined to the seedling phase and does not extend to the adult, although in heaths infection of the roots follows directly upon that of the seedling tissues.
The older theories of beneficial symbioses in mycorrhiza were deduced from distribution and structure untested by experiment. Modern research has been marked by the application of new and more precise methods and has revealed unsuspected relation ships. On the whole, it supports the view that the possession of mycorrhiza is frequently of service to vascular plants, the nature and degree of benefit depending upon the physical conditions of the environment and the character of the association in individual cases. (M. C. R.) BIBLIOGRAPHY.-The latest and most complete treatise, including a full bibliography, is that of M. C. Rayner, Mycorrhiza: An account of non-pathogenic infection by Fungi in Vascular Plants and Bryo phytes (London, 1927). See also H. Burgeff, Die Wurzelpilze der Orchideen (Jena, 1909) ; W. B. McDougall, "On Mycorrhizas of Forest Trees," Amer. Jour. Bot., vol. 1, p. 51 (1914) ; W. B. McDougall, "Mycorrhiza of Coniferous Trees," Jour. of Forestry, vol. ii., No. 3 (1922) ; E. Melin, Untersuchung fiber die Bedeutung der Baummycor rhiza (Jena, 1925) ; L. Knudson, "Physiological Study of the Symbiotic Germination of Orchid Seeds," Bot. Gaz., vol. lxxix., pp.