114
THE ESSEX NATURALIST
and, in general, even less conformable to pattern than the shoot
system. Trees, nevertheless, tend to produce one of three
types of root system, called tap-, flat- and heart-root respectively.
At least in the young tree in Oak and Silver Fir there is a tap-
root system, while a flat-root system, that is, a layer of roots
spreading out just below the surface of the soil, characterises
Spruce and Poplar. Either type of root system may occur in
Alder. In older trees of Birch and Beech several roots of more
or less equal thickness grow out from the base of the main
root, which itself disappears : a cluster of more or less equal
sized roots of this sort is a heart-root system.
The nature of the soil affects the root system and wet, compact
or oxygen-deficient soils tend to encourage shallow rooting.
Shallow rooting, although it makes the trees less wind firm, is
not necessarily detrimental to the vigour of the tree : the Oaks
of the Spessart district in Germany, which produce some of
the finest oak timber in Europe, are shallow rooted, while
Wibeck appears to have obtained satisfactory and conclusive
evidence that in Pines at least, bent, deformed roots gave better
stemmed plants at least for as long as 15 years, although he
reasonably argues that later the stability of the trees will doubtless
be impaired.
Roots do not necessarily last throughout the life of the tree;
they may die and decay, when their scars are sealed off by a layer
of cork.
In many trees there are long roots and much shorter ones,
the latter concerned with feeding and not long lived. There
is a strong tendency for the long roots to grow at the expense
of the short feeding roots and when this happens with a fruit tree
it may be necessary to root prune, that is to shorten all the roots
back to near the centre by cutting a trench round the tree and
severing the roots. It is often held that this has the effect of a
severe check on the tree which encourages fruiting at the expense
of leafing but the more likely explanation is that by shortening
the long roots the short feeding roots are encouraged to grow.
Similarly, in growing young trees from seed, frequent transplanting
is desirable in most species as this operation inevitably results
in the longer roots being broken and the appearance of a short
bushy root system.
So far we have considered the root as an underground organ,
but it is not always subterranean. We have seen how the Banyan
tree sends down adventitous roots from its spreading branches,
and in Britain the myriad adventitious roots on the stem of the
Ivy are used as climbing organs. In some trees the large roots
at the base of the stem spread out as supporting flanges and form
the buttress roots which are such a common feature of the trunks