Looking At Plant Stems

Core Practical 6 – From Topic 4 (Biodiversity and Natural Resources)

Aim

To identify sclerenchyma fibres, phloem sieve tubes and xylem vessels and their location within stems through a light microscope.

Equipment

Small piece of tinned rhubarb
Microscope slide
Coverslip
2 mounted needles
Forceps
Watch glass
Methylene blue (1% solution)
50% glycerol
Filter paper

Method

Place a small piece of tinned rhubarb on a watch glass. Use forceps to pick out one or two vascular bundles from this block of tissue and place them on a microscope slide.
Use mounted needles to tease the vascular bundles apart. Cover the tissue with a drop of methylene blue, and leave for 5 minutes.
Draw off the extra stain with filter paper. Place a drop of dilute glycerol on the fibres and mount under a coverslip.
Examine your preparation under low, medium and high magnification. If the tissues are not separated enough, place your slide on a piece of filter paper, put a filter paper pad on the coverslip and press down with your thumb. This may separate out the tissue. Do not move your coverslip sideways at all. You may need to re-irrigate the slide with glycerol after squashing it. To do this, place a drop of glycerol on the slide next to the coverslip. It will be drawn under the coverslip by capillary action. Blot off any excess and re-examine the slide.
Look for vascular bundles amongst the separated tissues.

Observations

The xylem vessels are empty, elongated tube-like cells; they may show different types of wall thickening, for example, spiral or annular (in rings) thickening or, in some cases, virtually complete lignification of the walls.

Phloem sieve tube elements are also elongated; they lack a nucleus even though the cells are alive with some cell cytoplasm. There are sieve plates with pores between adjacent cells. Each sieve tube element is associated with a companion cell.