In
woody plants, the phloem is
the inner layer of the bark. It
consist of sieve elements, parenchyma
cells, fibres and
sclereids.
Unlike xylem, the cells that make up phloem are living at maturity
and can carry materials both up and down the plant body. Phloem is
comprised of sieve elements, which are arranged end to end to form
passageways, and companion cells, which are closely associated with the
sieve elements even though their exact function is unknown. Mature
companion cells have both a nucleus and cytoplasm, while the mature
sieve elements contain only cytoplasm; for this reason it is thought
that the nuclei of companion cells may control the activities of
neighboring sieve elements. Phloem is responsible for distributing the
products of photosynthesis, such as amino acids and carbohydrates, from
the leaves to the rest of the plant.
Compare with
xylem. |