Without
fungi, there would be no forests, and perhaps not even any
land plants. This is because a few thousand fungi have evolved
intimate and essential relationships with the roots of almost
all living plants. They work like this...
The
fungus explores the soil and brings back scarce mineral
nutrients, especially phosphorus, to the plant. The plant
hands over to the fungus some of the sugars it has made
by photosynthesis.
The
fungi grow around and into the roots, establishing a microscopic
interface across which phosphorus moves one way while sugars
move the other way. Both partners benefit, and neither would
thrive without the other.
The
partnership is called a mycorrhizal symbiosis, and about
90% of all plants have these specialized fungi on and in
their roots.
There
are basically 2 kinds of mycorrhiza.
1) The
kind established with about 2 000 species of coniferous
trees involves about 5 000 fungi of the kind that form mushrooms
or truffles. These fungi produce a mantle around the roots
and grow between the root cells but never penetrate them.
This creates an ectomycorrhiza.
2) The
other kind is called an endomycorrhiza. This is a
very ancient relationship (it evolved 400 million years
ago) and involves microscopic fungi that penetrate the root
cells and make tiny tree-like organs inside. These fungi
form spores in the soil but no structures that could be
seen by the naked eye. Endomycorrhizas are formed by perhaps
250 000 species of plants.
For
pictures of both kinds look at Chapter
17. Much more detailed accounts can be found in the
book and CDROM.
