The green, green plants of home

3rd January 2003 at 00:00
Ray Oliver suggests secondary-level activities which reveal the absorbing nature of plants' relationship with sunlight

Everyone thinks that solar power represents the future. But it also represents the past - in fact, the entire history of life on earth. Everything we see growing or living around us is solar powered, depending directly or indirectly for its food on photosynthesis.

Activity 1: Can plants thrive equally well in different colours of light?

Sunlight contains all the colours seen in a rainbow. The light-absorbing pigments in leaves interact with some of this polychromatic light. One of them, chlorophyll, can absorb violet, blue and red light. The colour we see, green, is what is left over. It is a very successful piece of biochemistry, as we can see from the planet's green plant cover.

Ask the students to design an experiment to test this idea. They could use a selection of coloured filters made from Cellophane. They will need to decide what kind of evidence is required to show whether the plants are thriving. Is it the height of the seedlings, their starch content, leaf size or something else?

For a quicker result, use Canadian pondweed and monitor the rate that oxygen bubbles are produced using different colours of light. Some students will realise that differently coloured filters may transmit different light intensities.

Activity 2: How does chlorophyll act on the colour spectrum?

So, what colours do green plants need? The green colour of most plants suggests that leaves selectively remove blue and red light from sunlight. Pupils can demonstrate that the chlorophyll in leaves removes some colours from white light (between 430 and 680 nanometres).

Make a solution of chlorophyll as follows:

* Shred some green leaves into a mortar and add some sharp sand

* Cover the leaves with ethanol (alcohol) or propanone (acetone)

* Grind the mixture, then filter the green chlorophyll solution.

Now make a colour spectrum:

* Use a glass prism with a raybox or projector

* Focus the colour spectrum on to a square of white card

* Place the chlorophyll solution between the light and the prism. Try different-sized containers to test the effect on the spectrum

* Identify colours that have been subtracted from the original spectrum.

And now for the biological equivalent of watching paint dry...

Activity 3: How fast does your grass grow?

There are lots of ways students can monitor the rate of growth of grass or weeds:

* Measure the heights of 100 random specimens and repeat the sampling on future days. Display results on a histogram to emphasise the variation

* Cut grass stems at ground level and weigh them. (Pupils need to consider if the material should be dried first to avoid problems with variable water content.) Repeat on successive days

* Mow the grass in a fixed area, say five square metres. Weigh the grass cuttings (wet or dried?). Repeat in a similar area on successive days

* Extend the investigation to include plants such as sunflowers, cress, runner beans or bamboo.

Activity 4: How strong is grass or straw?

Dried grass and straw are versatile materials when woven or plaited for baskets, ropes, chair covers or even hats. Students can investigate some of the factors that determine the strength of such natural materials.

* Take a single strand of grass or straw and secure it at each end so it is horizontal

* Add weights in increments of, say, 10g to the centre of the strand until it breaks

* Plait together several strands and repeat

* Compare freshly cut stems with dried material

* Compare the strengths of round and flattened stems

* Compare complete stems with loose fibres separated from the stems.

Activity 5: Create leaf skeletons to reveal the pattern of veins What gives leaves their strength?

The reason why the thin green leaves of plants do not sag and droop is that they contain veins. This branching system of veins acts as a leaf support, rather like a skeleton. The veins connect with the sites on the leaves where photosynthesis is going on. The vein system supplies minerals and water and carries away the sugars produced in the leaves.

Students can study the patterns of veins in leaves from different plants.

* Make leaf skeletons by destroying the remainder of the leaf material. First boil the leaves gently for 30 minutes in a dilute solution of washing soda. Remove the leaves and leave them to soak in household bleach for 12 hours. Carefully remove, rinse and dry the leaf skeletons

* Display the leaf skeletons using an overhead projector, or attach them to an acetate sheet and place them on a window.

Activity 6: How big is the surface area of a leaf?

The vein pattern of one special leaf is believed to be an inspiration for the design of the Crystal Palace. The glass-and-iron building was the centrepiece of the Great Exhibition, held in London in 1851.

Joseph Paxton (1803-1865) had already designed a large glasshouse at Chatsworth House to grow a giant water-lily from the Amazon, Victoria regia, now called Victoria amazonica. The vein structure of the floating leaf made it strong enough to support the weight of his daughter, Annie.

Since photosynthesis occurs in the green leaves of plants, the size of the leaves is likely to be important for plant growth. Measuring the total leaf area of a plant seems a daunting task, but making an estimate is straight-forward. Ask the students to:

* Remove five representative leaves from the plant

* Place the leaves on graph paper and draw around them

* Calculate the leaf areas by counting squares

* Display the data on a histogram

* Count the leaves and so estimate the total surface area.

Before they carry out their measurements, you could ask the students to guess the plant's number of leaves and their total area. The guesses could be compared with the measured estimates.

You could also ask the students to compare a plant with a tree. To estimate the number of leaves on the tree, they could count the leaves on one branch and multiply by the number of branches.

Extension ideas 1. Investigate the connection between the rate of photosynthesis and the distance from a light source (light intensity).

2. Measure the effect of increasing the carbon dioxide available for photosynthesis. You could use a seltzer tablet to release the gas in an airtight container.

3. Find out if variegated leaves and non-green leaves also contain chlorophyll.

4. Research links between plant structures and architecture, such as in Greek-decorated columns.

This article meets these key stage objectives: Science at KS4 Sc2: green plants as organisms; Sc4, energy transfer; Technology at KS3, products and uses of materials. Note that risk assessments must be carried out before doing practical work with children.

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