Dicot Stem Cross Section

This is a cross-section (transverse section) of a typical herbaceous dicot stem. Its tissues are laid down in a characteristic manner.

EPIDERMIS
The outer layer, one cell thick, is the 'epidermis'. Some of these cells bear multi-cellular hairs or 'trichomes', which are beneficial for water conservation and defence. Because green stems can photosynthesise, the epidermis also contains 'stomata'. Just like leaves, the epidermis is covered with a waxy layer of cutin, the 'cuticle'.

CORTEX
The cortex comprises tissue located between the vascular blundles and the epidermis, often split into two layers. The outer layer is composed of collenchyma cells whose unevenly-thickened walls give flexible support to the growing plant. And, because it is a living material, can grow as the plant grows. The outermost collenchyma cells may contain chloroplasts, whose chlorophyll facilitates phososynthesis.

Below the collenchyma, the remainder of the cortex consists of unmodified parenchyma cells.

VASCULAR BUNDLES
In dicot stems, these are always arranged with the phloem towards the outside and the xylem towards to the centre. This has been described elsewhere (see 'Vascular Bundle Of Young Dicot Stem, Cross Section')

In dicots, the vascular bundles are arranged in a ring running the whole length of the stem, and connecting with the stele of the root and with the midribs of the leaf veins.

These bundles give much support, especially when a pericyle is present whose tightly-packed, lignified sclerenchyma fibres are extremely tough, forming a bundle, or cap, between the bundle and the cortex.

PITH
This is parenchyma packing material which forms the centre portion of the stem. In some plants, this is missing and the centre is hollow and often filled with water.

See also: 'Cross Section Of Young Dicot Root'

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'