Where present,
the bark most typically serves two very important functions.
The outer, mostly dead
tissues (outer bark) form a protective barrier between the plant axis and the
abiotic and biotic environment.
The inner tissue (secondary
phloem), including living cells (inner bark), is where sugar transport for the
plant occurs, and the inner bark also can have defenses against herbivores,
such as cells with tough cell walls (secondary phloem fibers or sclereids) or
cells and tubes filled with bitter or toxic chemicals.
Bark is a very
complex structure, consisting of cells that formed from lateral meristems
(cambia producing secondary tissues) but often containing some cells (living or
dead) that remain from the first-formed plant (primary tissues, such as
epidermis, cortex, and primary phloem).
In general, the inner bark is defined
very narrowly as the innermost sector consisting only of secondary phloem,
which is produced to the outside of and by the vascular cambium.
The outer bark
always includes all tissues formed by the lateral meristem known as the cork
cambium (phellogen) or a number of individual and often discontinuous cork
cambia.