ROOTS - The Soil - page 48

mycorrhizal fungi, and they perform many of the normal tasks of the root, especially
the absorption of nutrients from soil.
Roots live in company with a very large number of soil organisms, and for many
of them the environs of the root is the best part of the soil in which to find food. Roots
lose considerable quantities of materials into the soil, both as whole cells sloughed off
(for example from the root cap) and as chemicals that diffuse or are secreted from the
root. Something like 5-10 percent of all the carbon that is manufactured by the leaves
may end up in the soil in this way, even before the root dies. Since the root is the main
source of carbon, and hence energy, in the soil, the region of soil around the root re-
sembles an oasis in a desert. This region is called the rhizosphere, and in it develop
enhanced populations of bacteria, fungi and some of the smaller soil animals such as
nematode worms. Some of the fungi are quite innocuous, feeding on the organic sub-
stances lost by the roots; others are pathogenic and may invade the root, damaging it;
a third group form symbiotic associations with roots, and these are mycorrhizal fungi.
F IG. 16
An onion root showing VAM fungus (vesicular-arbuscular mycorrhiza). Entry point with longitudinally
running hyphae and arbuscules in the root cortex. Right VAM fungus in onion root showing vesicles and
hyphae in the root cortex. (From: J.M.Phillips & D.S.Hayman 1970.)
M YCORRHIZAS
The word mycorrhiza comes from two Greek words: mykes fungus and rhiza roots. A
mycorrhiza is a root infected with a particular type of generally beneficial fungus, but
there are several types of mycorrhiza involving different fungi and plants. The most
widespread and most ancient type, though not the most familiar, bears the cumber-
some name of vesicular-arbuscular mycorrhiza, almost invariably and understandably
abbreviated to VAM. The name derives from two structures formed by the fungus