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'

How Water Enters A Plant: Apoplast And Symplast Pathways

Water enters plant roots (along a concentration gradient) through fine root hairs (see 'Apoplast And Symplast Pathways In Root A Hair') then makes its way across the root tissues (cortex, endodermis and pericycle (see 'Cross Section Of Young Dicot Root') towards the xylem, that part of the vascular system which distributes water and dissolved mineral salts throughout the plant.

THREE ROUTES OF TRAVEL

Water may travel across the root by one of three pathways:

• apoplast - within cell walls only (unselective)
• symplast - in cytoplasm, through plasmodesmata (selective)
• cell-to-cell (transmembrane pathway) - through vacuoles of each cell (selective)

We have seen that a typical cell comprises an outer cell wall surrounding a partly permeable plasma membrane, enclosing cytoplasm and inner vacuole. We have also seen that the cytoplasm of adjacent cells is connected through 'gaps' in the cell wall called plasmodesmata.

APOPLAST PATHWAY
If once water enters the root hair, it travels from cell to cell, across the cortex, in the CELL WALLS ONLY, it is said to take the 'APOPLAST' pathway. Because cellulose is pervious and non-selective, both water and ALL dissolved minerals may be carried this way. Note that the plasma membrane (separates cell wall from cytoplasm) need not be crossed, not until the endodermis - a defensive ring of cells, one cell deep - is reached. So OSMOSIS need not be involved up to this point, only diffusion.At this point, the impervious Casparian strip blocks progress and forces both the water and mineral salts to cross the plasma membrane into the cytoplasm, where they can continue on the symplast pathway. However, not all mineral salts make it - the plasma membrane is selective and can filter out unwanted solutes.


SYMPLAST PATHWAY
If water passes from the cell wall through the plasma membrane to the cytoplasm, it can then travel from cell to cell through the plasmodesmata in the cell walls. This is called the 'SYMPLAST' pathway. Note that the plasma membrane, because it is partly permeable, can only be crossed passively by OSMOSIS. Passage of dissolved nutrients, because their molecules are too large, are blocked and cannot pass passively; they must cross the plasma membrane into the cytoplasm by active transport, a selective process; osmosis is not involved.

CELL TO CELL
Water can also enter the vacuoles (which are within the cytoplasm), then pass from vacuole to vacuole. This is called the 'CELL TO CELL' or 'TRANS MEMBRANE' pathway.

POINT OF INTEREST
It is commonly supposed that both water and minerals enter the plant cytoplasm as one process. They don't. Each enters independantly of the other; mineral salts can enter a plant even when no water is being absorbed. However, once inside the cytoplasm, minerals travel in solution.

TOXIC VEGETABLES
Although the endodermis prevents toxins and any other unwanted solutes from entering the stele of the plant, and hence travelling to the stems, leaves and fruits, these nasties may be present in the epidermis and cortex of the plant, taken in through the unselective apoplastic pathway. Since the cortex represents the bulky 'food' part of roots, we need to be extremely careful.

Apoplast And Symplast Pathways In A Root Hair

Plants take in water from the soil, along a concentration gradient, through uncutinized epidermal cells of their roots. Near the root tips (where most of the water is absorbed), these cells greatly increase their absorptive areas by extensions called root hairs. As water enters the cell (hair), it can take one of two routes depending upon whether or not it has entered the cytoplasm of the cell. These routes are called the APOPLASTIC and SYMPLASTIC pathways.APOPLASTIC PATHWAY
Because cellulose, the principal constituent of the cell wall, absorbs water like a sponge (think 'cotton wool'), water can travel from cell to cell - along with any solutes dissolved in it - via these cell walls WITHOUT crossing the plasma membrane to enter the cytoplasm of any cell. This is the APOPLASTIC pathway.

SYMPLASTIC PATHWAY
Water can also cross BOTH the cell wall AND the plasma membrane to enter the cytoplasm, then travel from cell to cell via the plasmodesmata in the cell walls. This route is called the SYMPLASTIC pathway. Because the plasma membrane is partly permeable, entry to the cytoplasm must be by OSMOSIS.

NUTRIENT SELECTIVITY

• There is a misconception that nutrients enter root hairs in solution. This is not strictly correct - it only applies to the apoplastic pathway, where the plasma membrane need not be crossed.
• However, in the symplastic pathway, both water and minerals (nutrients) must each cross the plasma membrane by a separate and independent process; one is PASSIVE the other ACTIVE.
• Water passes passively by osmosis.
• Minerals, on the other hand, must pass through the plasma membrane by ACTIVE TRANSPORT, a selective process.
  

See also: 'How Water Enters A Plant: Apoplast And Symplast Pathways'

Cross Section Of Young Dicot Root

This is a typical cross-section of a young dicot root taken in the region of root hair proliferation (see 'Plant Root Growth'), before lateral secondary growth has begun. Hence, the lateral meristematic tissue (cambium) is not yet present.

EPIDERMIS - Developed from the protoderm, it forms a cover, one cell thick, around the root's perimeter. In the region just back from the root tip, cells produce extensions as root hairs to increase the root's water-absorbing potential.

CORTEX - Developed from the ground meristem, these are typical 'fleshy' parenchyma cells with thin walls, large vacuoles and air spaces between cells, facilitating storage of food and water.

In roots of some plants (bog-type plants), some of the cortex cells are lost, producing even more air spaces 'aerenchyma', which allow gases to diffuse WITHIN the plant to and from the root tips.

ENDODERMIS - Surrounds the vascular tissue and can be seen as the innermost layer of the cortex. Unlike the epidermis, each cell wall contains a vertical ring of fatty suberin, called the 'Casparian Strip', making the wall impervious and thus keeping out unwanted solutes.

PERICYCLE - A layer one to several cells thick, between vascular tissue and endodermis. This is modified parenchyma which retains its capacity for cell division and produces the lateral roots which then punch their way out through the outer tissues.

VASCULAR TISSUES - PHLOEM AND XYLEM form an inner stele through the centre of the root, right up to the hypocotyl, where the root joins the stem. The structure then changes into that of a typical stem.