Geoscience Reference
In-Depth Information
inside the root: vesicles are globular storage bodies, and arbuscules are very finely
branched fungal strands (hyphae) where interchange of materials between host and
fungus occurs. Both are microscopic, less than a tenth of a millimetre across, and in-
deed VA mycorrhizas form no visible external structures, so that it is impossible to
tell if a plant has the association without microscopic examination of stained roots
family Endogonaceae, whose principal genera are
Glomus, Gigaspora, Acaulospora
and
Scutellospora
, and these fungi can only survive in association with the roots of a
plant. Their principal distinction is the size of their spores (
Fig. 17
), which are quite
enormous by fungal standards - in one species of
Gigaspora
they are over half a mil-
limetre across, compared to a typical figure for most fungi of around one-hundredth
of a millimetre; in most VAM fungi the spores are about a tenth of a millimetre.
F
IG. 17
Spores of the VAM fungus
Glomus invermaium.
(Photograph T.P.McGonigle.)
Although the fungi depend absolutely on plant roots, from which they obtain all
their carbon for energy, plants can in most cases grow quite well without them. Never-
theless, most plants that can form VAM do so under natural conditions, because the
fungus appears to offer a solution to an otherwise severe problem - the acquisition of
the essential mineral nutrient, phosphorus. Of the nutrients plants must obtain in large
quantities, phosphorus offers the greatest difficulty because it occurs in soil as phos-
phate ions which are so sparingly soluble that they move only very slowly through
soil. Once a root has used up the phosphate in solution in its immediate neighbour-
hood, it can only obtain more if other phosphate ions diffuse through soil from oth-
erwise unexploited soil nearby. This happens very slowly: a phosphate ion will nor-
mally move less than a millimetre through soil in a day. This must in fact have been
