Collins Mushroom Miscellany. Patrick Harding

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      Brown rollrim –Paxillus involutus

      {Martin Garwood/NHPA}

      Before Frank’s pioneering work, similar, short stubby rootlets, encased in a matted sheath, had been observed by American scientists trying to determine how ‘parasitic’ Indian pipe plants (which lack the green pigment chlorophyll and thus the ability to manufacture sugars) obtained their nutrients. In Britain the bird’s nest orchid, a woodland plant that also lacks chlorophyll, has been shown to form a parasitic relationship with a nearby existing tree/fungus mycorrhizal association. Bird’s nest describes the appearance of the orchid/fungus/tree roots, a feature that remains hidden underground.

      Trees rarely or never forming ectomycorrhizas include holly, yew and field maple, along with introductions such as horse chestnut and London plane. Such trees and a very wide range of shrubs and non-woody plants (herbs) form endomycorrhizas in which the fungal partner invades the root cells of the plant. The fungi that are involved belong to a group known as the Glomales. These differ from mushrooms and toadstools in that their hyphae lack cross-walls and very few species produce large fruitbodies. Partly for this reason, VA (vesicular-arbuscular) mycorrhizas, as they are now known, were largely ignored until the 1970s.

      It is now becoming apparent that VA mycorrhizas aid growth in trees that do not benefit from ectomycorrhizas. Discussing the implications of this, a leading researcher in the field observed ‘If a tree is not mycorrhizal, it is a dead tree’. VA mycorrhizas are also found in most herbaceous plants, including grasses and cereals. Here too the result is increased absorption of nitrogen and phosphorus by the plants. The transportation of phosphorus by fungal mycelia has been observed at rates as high as 0.5m per hour; a very efficient distribution network. This may necessitate a review of agricultural practices. The cultivation of soil destroys the mycelia that forms part of the beneficial mycorrhizas, not to mention the negative effects on the fungal partners of inorganic fertiliser additions.

      VA mycorrhizas are also found in mosses and ferns. Fossil relatives of the ferns, complete with fungus roots, have been found in rock deposits from 400 million years ago. It is probable that mycorrhizas helped plants to colonise the land and thus played an important role in the evolution of higher plants.

      In addition to increased nutrient absorption, plants benefit from mycorrhizas through an increased uptake of water. The ability of many plants to cope with climate change may be dependent on the presence of healthy mycorrhizas. Successful plant colonisation of soils of unusually high or low acidity, a common feature of land reclaimed after mining or industrial activity, is also dependent on the beneficial results of mycorrhizas. Additionally, mycorrhizas appear to give plants some protection from pathogenic fungi and parasitic nematodes.