Trees and all other green plants manufacture food materials from
minerals, carbon dioxide and water, deriving their energy from light;
and in the case of trees and shrubs, the unique material eventually
produced is wood. Almost all plants, including trees and shrubs, have
buds, leaves, flowers and fruit; but only trees and shrubs have wood.
The difference between trees and shrubs is simple. Trees have a single
woody stem, from which branches grow to form a crown. The branches of
shrubs arise at ground level, forming a crown without a stem.
The power of trees and shrubs to make wood is contained in a tiny layer
of cells, just below the bark, called the cambium layer. Every year the
cambium produces new wood on its inner side, as well as a layer of
tissue, called phloem, on the outside. At the same time, it reproduces
itself, to surround the ever-growing bulk of the plant. As long as they
live, trees and shrubs continue to grow all over. The trunk expands,
the branches thicken, the shoots lengthen. Growth only ceases when the
tree dies. Only then does the cambium finally stop making new wood.
In the growing season, life courses with ceaseless vigor through trees
and shrubs; the impression of stillness on the outside belies the
intense activity inside. Vast quantities of mineral-rich water flow
upwards through the new wood, from the roots to the highest leaves.
Sugar-rich sap descends through the phloem, from the leaves to all
parts of the tree. All this energy is expressed in the tree's growth,
in its flowers and fruit, in the seed which it produces in massive
quantities. In effect, every tree or shrub is one of life's richest
energy banks, storing food in its tissues, eventually returning
everything to the earth when it dies and decomposes, immeasurably
enriching the soil in which it stood rooted all its life.
Trees of Britain
Altogether more than 1,500 species of trees grow in Britain, and an
even greater number of shrubs; trees in productive woodlands alone
cover about 5 million acres of land. But of this great variety of
species, many are rare and only grow in botanic gardens and
collections; and only a tiny number of the trees and shrubs which grow
wild are truly native to this country.
Only 32 species of broad-leaved trees, three species of conifers and a
few shrubs were already established when the sea, swollen with melting
ice, swept through the Strait of Dover some 7,500 years ago, cutting
Britain off from the Continent and from the trees and shrubs which grew
there. The broad-leaved trees included many of today's most common
species, such as willow, oak, lime, ash and wych elm.. The coniferous
trees were yew, Scots Pine and juniper. All the other trees now growing
in Britain were introduced by man, either for ornament, timber or their
fruit.
