ROOTS - The Soil - page 52

Another remarkable feature of mycorrhizas that has recently come to light is
their ability to link plants together. When a root grows it may become infected by
mycorrhizal hyphae from older parts of the root or, more commonly, by encounter-
ing hyphae in the soil. These latter hyphae are likely to be part of the system radiat-
ing from another root, and so the two roots may become linked. Such links may be
from root to root in one plant, from plant to plant within one species, or even between
species. By labelling trees with radioactive isotopes, it has been found that materi-
als can pass from plant to plant by means of these links. At present it is unclear how
much can travel in this way, but there is intriguing evidence that seedlings establish in
swards more readily if they become mycorrhizal than if they remain uninfected, and
this raises the possibility that they may depend for their survival on nutrients trans-
ferred from neighbouring plants. If this turns out to be a widespread and important
phenomenon it may force us to rethink our view of plant communities: ecologists
have in the past tended to view them as dominated by intense competition between
plants; it may be that there is more co-operation than we thought.
There are other sorts of mycorrhizas. Some are obscure and little studied yet, but
two are especially worth examining: those on the roots of heathers and of orchids.
Heathers form associations with quite a different group of fungi from the ones we
have considered so far - the cup fungi, and in particular a fungus called Hymeno-
scyphus. The fungus is found in special fine roots called hair roots which are abund-
ant on the heather root system, and it infects almost all the cells of the root. Although
the details of this association are still somewhat obscure, it is clear that it acts in a
different way to the main types. The fungus appears to be able to break down organic
matter and to transport the nutrients it so obtains back to the host plant. VA and ecto-
mycorrhizal fungi do not seem able to do this. Certainly, the heather mycorrhiza as-
sists its host with nitrogen nutrition as much as with phosphorus, and it is probably no
coincidence that heathers are able to grow on extremely nutrient-poor soils through
this assistance. The heather mycorrhiza seems to be a special case that has evolved to
cope with such extreme conditions, and heathers are more dependent on their symbi-
otic partner than most plants.
The most remarkable case of dependency, however, is the orchid mycorrhiza.
All orchids are mycorrhizal, but here involving basidiomycetes again. Some of these
fungi are obscure if abundant (e.g. Rhizoctonia) , but others are well-known plant
pathogens. One of the most remarkable instances is the Japanese orchid Gastrodia
elata which forms a mycorrhizal association with honey fungus Armilariella mellea ,
which is a virulent pathogen of trees, much dreaded by foresters. Orchid seeds are
minute, weighing only a few millionths of a gram, and lack any differentiation into
root and shoot. They are virtually incapable of any independent growth and can only