Mycorrhizae, the symbioses between fungi and plant roots, are nearly universal in terrestrial plants and can be classified into two major types: endomycorrhizae and ectomycorrhizae. About four-fifths of all land plants form endomycorrhizae, whereas several groups of trees and shrubs, notably Pinaceae, some Cupressaceae, Fagaceae, Betulaceae, Salicaceae, Dipterocarpaceae, and most Myrtaceae form ectomycorrhizae. Among legumes, Papilionoideae and Mimosoideae have endomycorrhizae and usually form bacterial nodules. The members of the third subfamily, Caesalpinioideae, rarely form nodules, and one of the included groups, the two large, pantropical, closely related tribes Amherstieae and Detarieae, regularly form ectomycorrhizae. Nodules and ectomycorrhizae may well be alternative means of supplying organic nitrogen to the plants that form them.Those plants having endomycorrhizae usually occur in forests of high species richness, whereas those with ectomycorrhizae usually occur in forests of low species richness. The roots of ectomycorrhizal trees, however, support a large species richness of fungal symbionts, probably amounting to more than 5000 species worldwide, whereas those of endomycorrhizal trees have low fungal species richness, with only about 30 species of fungi known to be involved worldwide. Ectomycorrhizal forests are generally temperate or occur on infertile soils in the tropics. They apparently have expanded in a series of ecologically important events through the course of time from the Middle Cretaceous onward at the expense of endomycorrhizal forests.The invasion of the land by the ancestor of the vascular plants clearly seems to have been facilitated by the origin of symbiotic associations between these plants and certain "phycomycetous" fungi similar to those that are involved in endotrophic mycorrhizae at the present time (1-10). During the subsequent history of plants on land, additional kinds of fungus-plant associations have evolved in relation to the exploitation of different habitats and different population structures. The purpose of this paper is to review these associations in an ecological/evolutionary context and to explore the nature of the generalities that can be derived concerning them. Such relationships are particularly significant in view of the role of mycorrhizae in contributing to plant productivity and the consequent potential of manipulating such associations for human benefit (11). In pursuing the ecological and evolutionary patterns involved, we first review the characteristics of the different kinds of mycorrhizae.KINDS OF MYCORRHIZAE Endomycorrhiza. The fungus penetrates roots to form characteristic intracellular vesicles and arbuscles. Endomycorrhizae probably are regularly formed by about four-fifths
