Plant transport tissues - Xylem and phloem

Xylem

The xylem is a tissue which transports water and minerals from the roots up the plant stem and into the leaves.

Xylem consists of dead cells. The cells that make up the xylem are adapted to their function:

• They lose their end walls so the xylem forms a continuous, hollow tube.
• They become strengthened by a substance called lignin. Lignin gives strength and support to the plant. We call lignified cells wood.

Transport in the xylem is a physical process. It does not require energy.

Phloem

The phloem moves food substances that the plant has produced by photosynthesis to where they are needed for processes such as:

• growing parts of the plant for immediate use
• storage organs such as bulbs and tubers
• developing seeds

Transport in the phloem is therefore both up and down the stem. Sucrose is the transport sugar in the phloem. Transport of substances in the phloem is called translocation. Translocation requires energy as it is an active process.

Phloem consists of living cells. The cells that make up the phloem are adapted to their function:

• Sieve tubes - specialised for transport and have no nuclei. Each sieve tube has a perforated end so its cytoplasm connects one cell to the next. Sucrose and amino acids are translocated within the living cytoplasm of the sieve tubes.
• Companion cells - transport of substances in the phloem requires energy. One or more companion cells attached to each sieve tube provide this energy. A sieve tube is completely dependent on its companion cell(s).

Comparison of transport in the xylem and phloem