Transport of Materials from the Leaf
The leaf is the site of photosynthesis, where food chemicals are created for the whole plant. These substances need to be tranported to the parts of the plant which cannot make their own food. The chemicals are transported in phloem tubes.
Sieve tube elements (the cells which make up phloem tubes) are arranged in long columns. Unlike xylem vessels they are filled with cytoplasm, though they have no nucleus. The cell walls at each end of the phloem cell is perforated to form a sieve plates. The phloem cells have associated companion cells which do have a nucleus. The companion cell supplies the sieve tube elements with some requirements as the sieve tube element cannot make things like proteins on its own.
