ROOTS - The Soil - page 50

a problem to the very earliest land plants, since they had extremely poorly developed
root systems which would have been much less effective at exploring soil than the
highly branched root systems of modern plants. Remarkably, fossils of one of the
first land plants Rhynia , about 400 million years old, have fungi associated with their
rhizomes that appear almost identical to modern VAM fungi; it seems as though the
mycorrhizal symbiosis is a very ancient one. Certainly fossils of plants from Triassic
rocks (over 200 million years old) have clear VAM fungi in their roots.
Plants appear, therefore, to have faced a severe problem of getting phosphate
from soil at a very early stage in their evolution, and they seem to have solved it by
associating with fungi on several different occasions. VAM are found on the roots
of a wide range of plants, especially herbaceous ones, but also a number of woody
plants and trees. Families such as the Ranunculaceae, Rosaceae, Leguminosae, Labi-
atae, Scrophulariaceae, Compositae, Alliaceae and Gramineae all normally ocur with
VAM fungi in their roots. A few families are very rarely mycorrhizal, especially the
Cruciferae, Chenopodiaceae and Cyperaceae. About two-thirds of all plants appear to
be normally VA mycorrhizal.
There are several other types of mycorrhiza than the VAM, and they clearly
evolved quite separately. The best known, and the type which was first recognized, is
the ectomycorrhiza or sheathing mycorrhiza, characteristic of many forest trees, espe-
cially the Pinaceae (pines, spruces, larches, firs; see Plate 4 ) , the Betulaceae (birches,
alders) and the Fagaceae (oaks, beeches). This type of mycorrhiza involves more fa-
miliar fungi: almost all are toadstools, members of the Basidiomycetes ( see here ).
Some are well known and distinctive, such as fly agaric Amanita muscaria which
forms a mycorrhiza with birch, and is therefore almost always found growing under
birch.
Ectomycorrhizas were the first type to be discovered because they are visibly
different from uninfected roots. Ectomycorrhizal roots are stubby and often fork di-
chotomously, giving dense clusters. Each root tip is surrounded by a sheath of tightly
woven fungal hyphae and other hyphae radiate away from this into the soil. It is
much easier here than in the VA mycorrhiza to see how the symbiosis works. The fine
fungal threads penetrate the soil, picking up the immobile phosphate ions and trans-
porting them back to the sheath, where they are stored before being eventually passed
to the root. Meanwhile the fungal hyphae beneath the sheath, which are in contact
with the root cells, obtain sugar from them to feed the fungal tissues. It is easy to see
what the fungus gains: a reliable source of carbon for which it does not have to com-
pete with other soil fungi. At first sight the gain of phosphate to the plant is clear, too;
but why should plants use fungal hyphae to obtain phosphorus for them, rather than
growing new roots? The answer lies in the dimensions of roots and hyphae. A root is