ROOTS - The Soil - page 44

tions: gathering water and nutrients from soil, and anchoring the plant, though the lat-
ter may be fulfilled as a simple consequence of achieving the former in many cases.
There will be some thickening of the older members of the root system, since they
must supply and receive supplies from an ever greater number of growing tips and
so must have a greater transport capacity. In perennials, however, different consider-
ations apply. Anchorage becomes more of a primary function, especially where large
woody trunks develop above ground, and the root system may also become an im-
portant store for the plant's food reserves. Many biennials use much of their growth
in the first year to produce a massive underground store which is then drawn on for
the demands of flowering and fruiting in the following year. As a result, plants such
as turnip Brassica rapa , sugar beet Beta vulgaris , parsnip Pastinaca sativa and carrot
Daucus carota have been utilized over many years as food crops, and cultivated vari-
eties have been selected to have bigger tap- roots.
Perennials, too, store reserves in swollen roots, probably largely because below-
ground stores are less vulnerable to predators than those above ground. The solid mat-
rix of the soil has precluded the evolution of a subterranean cow. Nevertheless, these
underground stores are liable to be eaten by a large range of invertebrates and smal-
ler vertebrates, and they are frequently protected by toxic compounds produced by
the plant as anti-herbivore defences. One of the best known of these is the root of
the giant yellow gentian of the European mountains, Gentiana lutea , which is used to
make a strong-flavoured drink, reputed to have marvellous medicinal qualities. Other
familiar, strong-flavoured roots include horse-radish Armoracia rusticana , arnica Ar-
nica montana and dandelion Taraxacum; what is a strong flavour to us and may have
pronounced physiological effects, such as the diuretic action of dandelion, is probably
lethal and certainly a powerful feeding deterrent to some small animals.
Perennial plants are of two kinds: those that produce new growth each year but
die back and pass the winter (or other unfavourable season, such as a dry summer)
as some such structure as a bulb, corm, rhizome or rootstock; and those that retain
at least some of the previous season's growth and build on it the next year. A blue-
bell and an oak tree are both perennials and may both survive a very long time, but
though a bluebell may spawn many new bulbs vegetatively during its lifetime, none
of them ever produces anything bigger than a bluebell; an oak tree gets larger each
year. A similar distinction exists below ground. The bluebell produces a new set of
adventitious roots each year from the base of the bulb, whereas an oak tree has a per-
ennial root system. Just as the branches of a tree become woody and thicker with age,
and bear short-lived leaves, so the roots become thick and woody and bear short-lived
rootlets. The main below-ground structure of a tree is just as permanent as what we
see above ground, and the parallel even extends to the fact that some woody plants