ROOTS - The Soil - page 42

CHAPTER 3
ROOTS
Virtually all flowering plants and ferns have roots, except for a few floating plants,
such as the duckweed Wolffia arrhiza , and some parasitic plants, such as dodder Cus-
cuta , though these have structures which penetrate their hosts and act rather in the same
way as roots. Some plants have very limited root systems, especially the non-green
orchids such as bird's-nest orchid Neottia or ghost orchid Epipogium , though they are
really a special case of parasitism, as we shall see.
As with so many aspects of biology, it is much easier to recognize a root than to
define it. The most familiar type is the root that develops from a germinating seed and
branches more or less profusely, but many plants have adventitious roots that arise in
dense clusters from the base of the stem, or even from leaves. The long, little-branched
roots of creeping buttercup Ranunculus repens or arrow-head Sagittaria sagittifolia ,
the roots that spring from the base of a willow branch kept in water, and even the roots
that form on the edge of a leaf of cuckoo flower Cardamine pratensis if it is pressed
onto moist earth, are all adventitious roots. Figure 14 illustrates both types of root.
A seed of a typical herbaceous plant, such as clover or lettuce, has a preformed
root already present. It is usually this root that first breaks through the seed coat and
initiates germination. The young root has a growing point or meristem at its tip, pro-
tected by a cap of loosely adhering cells. The meristem continually produces new cells,
and immediately behind the tip these start to elongate, pushing the tip through the soil.
In the process, cells of the cap are broken off by soil particles, but again they are re-
placed by the meristem. In some species, such as apple, the cap has a distinct point
which may improve its ability to penetrate the soil, but in others it is more rounded.
All the growth of a root system, except for radial expansion of the older roots,
takes place in this tiny region, a few millimetres long, just behind the root tip. The rest
of the root system is therefore unable to change its position and so a good distribution
of roots, with respect to the pattern of water and nutrients in soil and to other roots,
depends on the ability of the root tip to grow in the best direction and at the best rate.
The root tip can sense the conditions of the soil it is growing through and can alter dir-
ection, for example to avoid dry or toxic soil, rather like a miniature sense organ. The
other important piece of root behaviour in this respect is branching. Behind the zones
of cell production and elongation, there is first a zone in which the root produces hairs,
tiny excrescences from the surface cells, most often only 1 or 2 millimetres long, which
help to maintain contact between root and soil and to increase the surface area for the