116 THE ESSEX NATURALIST
Gardeners will recall the advice in pruning—cut close to the
bud, not well above it—the reason will now be obvious.
Similarly, shaded branches may not renew their cambial activity,
and when this is so the branch dies.
The phloem, which constitutes the inner part of the bark, is
also added to, but much less rapidly than the xylem. Neverthe-
less, one might expect in the course of years to see a big increase
in the thickness of the bark as a result of phloem formation,
in the same way as is apparent in the wood ; but this increase is
rarely very great, since the outer surface of the bark is continually
being shed.
Increasing girth of the stem and root gives rise to another
problem, which is here considered only for the stem. The
protecting skin of the young stem does not, normally, increase in
girth with the expanding inner tissues, and as a consequence is
split: the cortical tissues would thus be exposed to the exterior,
permitting loss of water from the cells as well as the entrance of
harmful fungi, were not a new protective layer formed. This is
the cork or outer bark. Cork also arises from a secondary
meristem, which forms somewhere between the epidermis and
the phloem. In the Willows cork formation occurs in the
epidermis itself, but usually it takes place somewhere in the cortex.
This cork cambium may expand with the growing trunk, when a
smooth bark is produced, as in Beech and in young Birch trees,
but usually it suffers the same fate as the epidermis and successive
cork cambia arise, not necessarily as continuous sheaths, but
often as separate patches. The new cork cambia arise nearer
and nearer to the phloem until, finally, when all the primary
tissue outside the phloem has been used up, cork is produced
in the phloem itself. The formation of successive cork cambia
leads to cracking of the outer bark, as is seen in Elm and old Birch.
The outer layers of bark are gradually shed and the shape
of the shed bark depends upon the structure of the cork : some-
times where there are thin-walled and thick-walled cork cells
in alternating layers, it comes off in thin paper flakes as in Birch.
There may be definite abcission layers in the bark as in the
Planes, where exfoliation is in the form of rather thick plates or
again it may slough off in the form of strings, a phenomenon
seen in some of the Eucalyptus trees which are, appropriately,
called stringybarks. In some trees, like Beech, the bark comes
off in quite small particles.
Again some trees are characterised by a thin bark: such a
tree is Spruce, in which there is a rapid shedding of the outer
layers. Perhaps the outstanding examples of the other extreme
are in the Sequoias, where the bark may be as much as one to two
feet thick in old trees.