SummaryThe fine structure of one of two common types of mycorrhiza formed by Pinus radiata D. Don is described. The results showed inter alia that the ability of a fungus to form mycorrhiza depended on its tolerance to polyphenolic compounds in the so-called tannin layer of the cortex. Hartig net formation is a process in which the invading fungus lysed the middle lamellae and then separated the cells of the host by mechanical action. Evidence of the transfer of carbohydrate from the cortical cells of the host into the fungus in the Hartig net was obtained. Typical dolipore septae were observed both in the mantle and Hartig net hyphae. A new and hitherto undescribed structure was observed in the fungal partner.

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Scanning electron microscope (SEM) studies of calcareous soils and calcretes from South Australia reveal a fossilized community of soil micro-organisms dominated by filamentous structures preserved in fine detail by calcite. In the various calcrete lithological facies, the filaments form dense mats within channels and voids, and also occur within the matrix where they are intimately associated with micrite. The calcite forming the filaments has a variety of crystal habits: the nature of the microcrystals is specific to each filament but varies significantly between adjacent filaments. In the calcareous soils there are various stages between the primary filaments and the calcite encrusted structures characteristic of the calcretes, suggesting that in vivo biochemical processes dominate the mechanisms of calcification. This hypothesis is supported by the specificity of the habit of calcite microcrystals on each filament. It is suggested that the organisms deposit calcite microcrystals within the mucilaginous sheath or in the cell wall (or both) as a detoxification mechanism in response to their highly calcareous environment. Based on the identification of structures resembling fruiting bodies, at least some of the filaments appear to have been fungal hyphae, which are known to be responsible for stabilizing macroaggregates in soils. Calcified filaments may produce permanently stabilized macroaggregates which provide the locus for further carbonate precipitation, leading to eventual induration of the soil.

The tylosis wall in Eucalyptus obliqua L'Herit. is shown to be composed of two microfibrillar layers. The outer layer (T1), with randomly orientated microfibrils, is covered with amorphous granular material. The inner layer (T2) is multilamellate. In sclerosed tyloses of E. miniata A. Cunn., each lamella of T2 is composed of many layers of microfibrils. Simple pits, delineated by circumferentially orientated microfibrils, are found in both sclerosed and non-sclerosed tyloses. The tylosis in E. obliqua is shown to arise from a two-layered structure formed within the secondary wall of the ray cell. This layer extends into the pit chamber, covering the pit membrane on the ray side. Following the breakdown of the vestures and the pit membrane, this double layer bulges out into the vessel to form the tylosis.

SummaryTwo common mycorrhiza types of Pinus radiata were examined by light and electron microscopy. Large numbers of bacteria and fungal species other than those forming the mycorrhiza as well as diatoms were observed in the mycorrhizosphere. Different morphological types of bacteria were characteristic of different mycorrhizal types, and in some cases the bacteria were associated with lysed regions of the mantle. The distribution of the bacteria within the rhizosphere is discussed in relation to the clay minerals and the carbohydrate and polyphenol metabolisms of the host.Within the mantle and the Hartig net, the hyphae have a structure typical of basidiomycetes, but within the tannin layer the hyphae have an abnormal distorted appearance. This is correlated with changes in cytology and is construed as evidence of the production of polyphenols toxic to fungi.. A possible role of these polyphenols in the control of mycorrhizal associations by selective action on soil fungi entering the rhizosphere is discussed.