Mycorrhizal associations are believed to have been a significant contributing factor for land colonization by plants, yet to date only one endomycorrhiza has been described from Devonian floras. We report a new occurrence of this association in an axis of a putatively primitive member of the Cladoxylales that is preserved in a carbonate cobble from the Lower Devonian Battery Point Formation, Gaspé, Québec. Stem anatomy consists of a partially dissected protostele with radially bifurcate primary xylem ribs. The inner cortex contains parenchyma and cells that appear very similar to transfusion tracheids; the outer cortex is composed of thick-walled fibers. Fungal hyphae are found throughout the inner cortex in a typical Arum pattern. Vesicles are common, with concentrations in localized areas of the cortex; each is elliptical, non-septate, and bilayered, with a basal hypha continuous with the outer vesicle layer. Extraradical spores arise from intraradical hyphae through stomatal openings or are disarticulated in the matrix. Arbuscules have short, broad trunks that narrow abruptly to branches, although this appearance may be artifactual due to arbuscular degradation. Arbuscules are confined to a distinct layer 7-10 cells thick, only a few cells removed from the main vascular tissue and in conjunction with the transfusion tracheids. The fungus is comparable to Glomites in several respects; differences include the location of the arbuscular zone relative to the main vascular tissue and the structure of the vesicles. This occurrence suggests that mycorrhizae were cosmopolitan by the early Devonian, supporting the symbiotic theory of land plant origination.

Key words: Cladoxylales, Devonian, Glomales, mycorrhizae