ABSTRACT
Depending upon whether the bulk of the fungus is outside or inside the root, mycorrhiza is broadly divided into ectomycorrhiza (EM) or arbuscular mycorrhiza (AM), respectively. Most fruiting structures like mushrooms and puffballs are produced by EM fungi (Figure 5.2). These have a conspicuous mass of hyphae coating the root, between the cortical cells of the root, and outward, spreading considerably into surrounding soil and litter. In a cross section of root, the hyphae are seen penetrating the cortex and forming a thick sheath, with an intercellular network of hyphae that develop between root cells, called the Hartig net. Intracellular hyphal coils formed within the
epidermal cells are called pelotons. The ectotrophic mycorrhiza can connect plants within communities and exchange resources through a common hyphal network. Orchids that have insufœcient photosynthesis gained 6-14% of mass when linked to an autotrophic plant but lost 13% when not linked (Simard et al., 1997). Most trees have arbuscular mycorrhiza in which the fungal hyphae ramify into tree-like structures called arbuscules within the root cells, invaginated by the host plasma membrane. The arbuscules provide a large-surface symbiotic interface for exchange of mineral nutrients from the fungus to the plant. Regardless of the mycorrhiza category, fungal hyphae breach cell walls but remain separated from the cell cytoplasm by a plant-derived membrane.
