THE LIVING TREE 113
which flower and fruit on spur shoots, like Apples and Pears,
and in which long shoots are pruned back to encourage the growth
of the spur shoots. It might be as well to add a fourth class,
viz. plants which should not be pruned or at least only pruned or
thinned with the greatest care and restraint: in this class will
come the Plums and Cherries, in which pruning encourages the
growth of spur shoots so that they become long and non-fruit
bearing branches : pruning here encourages rank vegetative growth
at the expense of fruit : but perhaps what has just been written
above on pruning has been over-simplified. Modern practice is
not to prune back hard, in, for example, Apples and Pears, but to
keep down old growth and encourage the growth of new wood; to
keep, as it has been put, a continuous fountain of new wood,
that is, to cut away the less virile parts of the tree and to encourage
the growth of vigorous young wood.
Some trees are what might be called self-pruning, ridding
themselves of branches periodically. This phenomenon—
cladoptosis, to give it its technical name—is an annual pheno-
menon in the Swamp Cypress (Taxodium distichum (L.) Rich,), one
of the few deciduous conifers. If we walk under a Swamp Cypress
tree in autumn we notice, not so much the carpet of fallen leaves,
but the mass of small twigs which has fallen with the leaves.
This shedding of the smaller branches is quite characteristic of
the tree and the phenomenon is also well shown by our native
Oaks, where the cast branches separate by means of neat ball
and socket joints. Normally, such branches are not large, but
naturally shed shoots as much as 3-31/2 feet long are recorded for
a Cottonwood, while the Durmast Oak has been found to shed
branches 21/2 feet in length.
Much more familiar than the fall of the branches is the
fall of the leaves of deciduous trees. At the base of the leaf
a weak layer—the abscission layer—is formed, and by means of
this the dead leaf separates easily from the stem. But leaf-fall
would be a serious matter for the tree were the scar of the fallen
leaf to remain thus, since it would be an open wound which would
form a means of entry for fungus parasites : to guard against this
contingency the leaf scar is sealed over with a protective layer of
cork.
So far little mention has been made of the underground
part of the tree. Like the shoot system, the root system is of
indefinite growth, continually increasing in length by means of
its apical meristems. These growing points, being in more
equable conditions below ground, are less in need of protection
than the stem apices ; nevertheless, they are protected against
hard particles in the soil by the root cap, which fits over each
root tip like a finger stall. The root system is very extensive