Vascular Bundle Of Young Dicot Stem, Cross Section

This is a transverse section (cross-section) of a typical dicot stem vascular bundle. These are comprised essentially of phloem and xylem tissues which provide both conduction and support. In dicot stems, these bundles are arranged in a ring (see 'Dicot Stem Cross Section') which is separated from the epidermis by cortex tissue. Bundle vascular tissue reaches throughout the whole plant and forms characteristic patterns depending upon whether it is in the root, stem, or leaf. Only in the stem and leaf is it referred to as 'bundles; it forms the 'stele' in the root.

PERICYCLE
Although the xylem tissue's cell walls are supportive, further support is often given to dicot stems by a sheath of sclerenchyma fibres (the 'pericycle', 'bundle sheath', or 'phloem fiber cap') which sits above the phloem tissue. Although not present in all dicot stems, it allows herbaceous stems to flex easily in the wind without breaking.

PHLOEM TISSUE
Consists of sieve tubes, companion cells, phloem parenchyma and phloem fibres (see 'Plant Tissues'.

In vascular plants, phloem tissue carries manufactured food from sites of photosynthesis (mostly in the leaves) to non-photosynthesising parts (mostly the roots). It also carries food products from storage in the roots to any other part where it can be utilised. This two-way traffic (up and down) differs from xylem transport, which is only upwards.

Almost all the sugar is transported in the form of the carbohydrate sucrose (a disaccharide), because this is both relatively inactive and also highly soluble (so can be carried in high concentrations).

Phloem also transports certain processed minerals, particularly nitrogen and sulphur in the form of amino acids, phosphorus as phosphates and potassium ions. Trace elements and growth hormones are also carried.

VASCULAR CAMBIUM
This is a thin ring of cells separating the outer phloem from the inner xylem. It is responsible for secondary growth in older stems.

XYLEM TISSUE
Dead material consisting of thick,lignified (woody) material. The 'metaxylem' is easily identified by the large mature xylem vessels, fibres and parenchyma. The 'protoxylem' is spirally or annually thickened and is closer to the centre of the stem.

Xylem tissue conducts water and nutrients upwards from the roots to the rest of the plant. Analysis of xylem sap has shown that much of the nitrogen is not carried as nitrate or ammonium ions, but as manufactured amino acids. This must therefore have occurred in the roots. Some organic phosphates have also been found.

See also: 'Cross Section Of Young Dicot Root'