FIGURE 3.43 Protoascon missouriensis thick-walled (a)zygosporangium (AZ) (Pennsylvanian). Bar = 50 |m.

multilayered spores which are attached to non-septate hyphae. More than 90% of extant land plants have a symbiotic (mutualistic) relationship with mycorrhizal fungi in their roots. There are two basic types of extant mycorrhizae: ecto- and endomycorrhizae. Endomycorrhizae are formed by members of the glomeromycetes and are the most common form today. The fungal hyphae grow within the host root, and although they penetrate the host cell walls, they do not penetrate the plasma membranes. Most produce arbuscules, highly branched hyphal structures that provide for exchange between the fungal symbiont and its host. Some also produce storage organs called vesicles (the vesicular-arbuscular mycorrhizae). Ectomycorrhizae are formed by members of the Basidiomycota and a few ascomycetes; they are less common today and occur primarily in woody plants of the temperate zone, including many conifers (Chapter 21). In this case, the fungal hyphae form a net around the outside of the plant root, which penetrates between the cells of the root itself. Although there are many hypotheses for the establishment of this fungal-land plant association (e.g., resistance against drought, defense against root herbivory, etc.), the mutualistic association provides for increased mineral nutrient uptake by the plant in exchange for a source of carbon for the fungus.

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