ROOTS - The Soil - page 55

of such conditions, for they possess an enzyme, nitrogenase, which can perform the
feat at normal temperatures and pressures. One essential, however, is an atmosphere
low in oxygen, and this is achieved in the nodule by the plant producing a form of
haemoglobin, the pigment found in blood. Just as in blood, the leghaemoglobin (or
legume haemoglobin) combines with oxygen, and so protects the enzyme. As a result,
effective N-fixing nodules are a distinctive pink colour. The appearance of this pig-
ment in both mammalian blood and legume root nodules is a remarkable instance of
evolutionary processes achieving similar solutions to analogous problems.
In many respects the nitrogen-fixing symbiosis works like the mycorrhiza. The
bacterium fixes nitrogen from the air and exchanges it for sugars from the plant.
Plants benefit from this if the supply of nitrogen from soil is limiting their growth
and the loss of sugar to the bacterium is therefore less deleterious. In fertile soils,
the extra nitrogen is less valuable and the lost sugar could have been used for more
growth, so that legumes lose their advantage and are rare on such soils. Legumes are
not the only plants that can fix nitrogen from the air in this way but they are the only
plants to use Rhizobium , except for one tropical shrub in the elm family. Other nitro-
gen fixers generally form associations with a different microorganism, an actinomy-
cete called Frankia. Whereas the Rhizobium association is with a single plant fam-
ily, the Frankia associations have evolved with an apparently random assemblage of
plants, including alders Alnus , sea buckthorn Hippophaë , mountain avens Dryas and
bog myrtle Myrica gale. Altogether, nearly 200 species in 14 quite unrelated genera
have evolved this habit. At first, it seems extraordinary that such a beneficial trait
should not be more widespread, or alternatively that those plants that have picked up
the ability should not be more abundant, since they would appear to have a great ad-
vantage over their competitors. In practice, it seems that the benefits are only really
great in very nitrogen-poor soils; nitrogen-fixing plants can be abundant on materi-
als that have been deposited by glaciers or on rock debris, for example. One of the
first colonists of glacial moraines in many parts of the world is alder, and, in the Ca-
nadian Rocky Mountains, the dwarf undershrub Dryas drummondii , a relative of the
widespread mountain avens D. octopetala. Probably this is what happened in north-
ern Europe after the retreat of the last glaciers of the most recent glacial period, some
12,000 years ago. Peat and lake deposits from that period contain a very high propor-
tion of Dryas pollen. Similarly, broom and gorse appear abundantly on such nitrogen-
poor materials as the sand heaps left behind by china clay mining in Cornwall (see
chapter 10 ) .
These plants, however, contain the seeds of their own destruction. As they grow
and fix nitrogen from the air with the aid of their microbial partners, so they continu-
ally add it to the soil, as their leaves and roots die and decay. In a short time, perhaps