(1) The range of mycorrhizal types is briefly compared, with respect to structure and nutritional mode of the symbionts. Ectotrophic, vesicular-arbuscular and erica-ceous mycorrhizas of autotrophic plants are selected for further consideration, both because the symbionts have nutritional similarities, and because recent experimental work provides a basis for useful comparisons.
(2) A generalized, qualitative model of interactions between the symbionts is presented, with the aim of providing a framework for discussion of the similarities and differences between the mycorrhizas of autotrophic plants. The model describes the distribution of biomass and the flow of carbon and mineral nutrients, together with the effects of distribution of fungal inoculum and environmental conditions.
(3) Experimental work pertaining to the model is discussed with emphasis on experimental problems, growth depressions, changes in root: shoot ratio and nitrogen nutrition, as well as the more frequently discussed increases in growth and improved phosphorus nutrition.
(4) Nitrogen is considered with respect not only to its uptake, but also in relation to the possible involvement of mycorrhizas in pH regulation and the absorption of cations by plants.
(5) The importance of mycorrhizas in forestry and agriculture is briefly discussed.
(6) It is concluded that more research into the physiology and ecology of mycor-rhizal associations is required, in order to provide a basis for effective management of the symbioses in agricultural and natural ecosystems.