We will now examine the complex permanent tissues. Remember the difference between simple and complex permanent tissues is that simple permanent tissues are made up of cells of the same type whereas complex permanent tissues are made up of more than one cell type that combine to perform a particular function. We will examine the vascular tissues, xylem and phloem tissues next.
Xylem has the dual function of supporting the plant and transporting water and dissolved mineral salts from the roots to the stems and leaves. It is made up of vessels, tracheids, fibres and parenchyma cells. The vessels and tracheids are non-living at maturity and are hollow to allow the transport of water. Both vessels and tracheids have lignin in their secondary walls, which provides additional strength and support.
Xylem vessels are composed of a long chain of straight, elongated, tough, dead cells known as vessel elements. The vessel elements are long and hollow (lack protoplasm) and they make a long tube because the cells are arranged end to end, and the point of contact between two cells is dissolved away. The role of xylem vessels is to transport water from roots to leaves. Xylem vessels often have patterns of thickening in their secondary walls. Secondary wall thickening can be in the form of spirals, rings or pits.
Tracheids have thick secondary cell walls and are tapered at the ends. The thick walls of the tracheids provide support and tracheids do not have end openings like the vessels. The tracheids’ ends overlap with one another, with pairs of pits present which allow water to pass through horizontally from cell to cell.
In addition to transporting water and mineral salts from roots to leaves, xylem also provides support to plants and trees because of its tough lignified vessel elements.
|Long cells||Form effective conducting tubes for water and minerals|
|Dead cells: no cytoplasm||No obstruction to water transport|
|Thick, lignified walls||Support the plant and are strong enough to resist the suction force of transpiration pull, so they don’t collapse|
|Pits in cell walls||Allow lateral water transport to neighbouring cells|
|Tracheids have tapered ends||Improved flexibility of the stem in wind|
|Vessels elements have open ends||Water is transported directly to the next cell|
|No intercellular spaces||Added support for the stem|
|Living parenchyma cells in between xylem||Form vascular rays for water transport to the cortex of the stem|
|Patterns of secondary wall thickening||Improve flexibility of the stem in wind and allow the stem to stretch as it lengthens|
Investigation: Observing the Patterned Secondary Walls in the Xylem of Fresh Plant Tissue
To observe the patterned secondary walls in the xylem of fresh plant tissue
- celery stalk, rhubarb stalks or pumpkin stems (macerated – chop them across and boil them in water for 3 minutes, then add an equal amount of glycerine. Cool before using. It can be stored for a few months in the refrigerator.)
- microscopes and slides
- dissecting needles
- petri dishes or watch glasses
- eosin solution
- Lift a small piece of celery / any other tissue chosen from the dish and transfer it to a watch glass or petri dish.
- Use the dissecting needle and a pencil to tease the tissue apart (separate the thread-like, thicker cells away from each other). Try to get the long cells away from each other, otherwise bundles will be too thick to allow you to see individual cells. Ignore the thin walled parenchyma cells around them.
- Transfer the plant tissue to a microscope slide and add eosin solution. Separate a bit more if necessary.
- Examine under low power, focusing on the bundles of xylem vessels. Look for long bundles of fairly wide cells with thickening in the form of rings or spirals. Do not confuse xylem vessels with the more common and much narrower sclerenchyma fibres – fibres have walls all the same thickness, have no spirals or rings and they are pointed at the end. If necessary, make a second slide if you did not find xylem.
- Move a good part to the centre and enlarge. Examine the secondary walls of these cells.
- Describe the shape of xylem vessels.
- What secondary walls patterns do you see?
- Suggest the function of such secondary walls.