The diversity of mycorrhizal structures applies not only to the plants and fungi involved, but also to the structural and functional details of the association. In this bewildering assembly of disparate, often only superficially connected phenomena loosely called "mycorrhizal associations," Harley's (1969) book The Biology of Mycorrhiza has been a beacon of factual information to specialists as well as ecologists, soil microbiologists, and others with peripheral interest in the subject. To create some order, mycorrhizal associations are usually divided into four main groups: ectotrophic or sheathing mycorrhizae occurring chiefly in temperate tree species and in Eucalypts; vesicular-arbuscular (V A) mycorrhizae occurring in most annuals, herbaceous perennials, temperate and tropical fruit trees, and most tropical timber trees; mycorrhizae of the Ericales; and mycorrhizae of the Orchidaceae. A range of other "mycorrhizal" associations have been described, notably in the Pteridophytes (Boullard, 1958) and Hepaticae (Stahl, 1949), but there is little information on these beyond their description.