The Arum-Paris continuum of mycorrhizal symbioses.

•  A survey of 12 plants colonized by six species of arbuscular mycorrhizal fungi was conducted to explore the diversity of Arum and Paris mycorrhizal structures. •  Surveyed root material was sectioned both longitudinally and transversely, double-stained and mycorrhizal structures were identified. A detailed time course experiment using four plant, and four fungal species, was used to investigate the sequential development of hyphae, arbuscules, hyphal coils, arbusculate coils and vesicles. •  The survey indicated that there was a continuum of mycorrhizal structures ranging from Arum to Paris, depending upon both the host plant and the fungus. The time course showed that total colonization increased, and that the establishment of the various mycorrhizal structures did not appear to change greatly over time. •  It was concluded that identification of fungal structures and their subsequent development into morphological types is not easily defined. Visual inspection of root squashes is not always adequate, especially where transverse sections are needed to determine if longitudinal hyphae are inter or intracellular; this is essential to distinguish intermediate types.