Exploring horse chestnut flowers

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Even at the bud stage of horse chestnut (Aesculus hippocastanum), you can clearly see developing leaves and flowers and by mid-May to early June, horse chestnut trees are normally in full flower. It is a spectacular sight with many thousands of flowers in large pyramidal inflorescences; often known as ‘candles’.

KS2 and KS3 pupils can find plenty of interest exploring these flowers. Using their knowledge of flower structure and function they can investigate aspects of pollination, make some predictions, collect data and try to find reasons for some of their observations. Horse chestnut flowers show variation in the number of floral parts and also in the petal colour – the ‘blotches’ on the petals range from yellow through salmon pink to a pinkish-red (carmine). Many of the flowers function as ‘male only’ flowers – the female parts (gynoecium) remain undeveloped. (Pupils described these as ‘miserable looking’!)

You can download the annotated drawings of a male horse chestnut flower on the right-hand side of this page. 

At Juniper Hall Field Centre, we have used horse chestnut flowers as a basis for investigation with a number of groups of pupils. Some of the ideas which developed and data collected from two such groups are given here.

First pupils were given a single horse chestnut flower so that they could work out its structure. They noticed the variation in colour and tried to find reasons for this. One group suggested that the pinkish (red) flowers were older than the yellow ones. They had also noticed that when the flowers first open, the petal blotch was yellow and the stamens turned downwards, but as the flower developed, the stamens began to turn upwards and shed their pollen. [It is known that insect pollinated flowers may change their colour when the have been pollinated, or they run out of pollen or other insect attractants, so that they cease to attract insects. (see Proctor et al)]

From these observations, the pupils predicted that ‘the red flowers will have fewer turned-down stamens than the yellow ones’. To test their prediction, they examined 144 flowers (5 to 10 for each pupil). In each flower, they recorded the number of stamens that turned up and the number turned down as well as the colour of the petal blotch. Because the number of stamens per flower varied, they decided to give numbers of stamens up or down as a percentage of the total number of stamens. Some individual results as well as combined class results are given in Tables 1 and 2.

Colour of blotches
Number of stamens
Up Down
 

Salmon pink

 

7
 

0
 

Yellow

 

5
 

3
 

Red

 

7
 

0
 

Salmon pink

 

7
 

0
 

Yellow

 

0
 

6
 

Yellow

 

3
 

4
 

Salmon pink

 

6
 

0
 

Yellow

 

0
 

8
 

Yellow

 

8
 

8
 

Red

 

0
 

0

Table 1 One pupil examined 10 flowers. In each flower she recorded the colour of blotches on the petals and whether the stamens were up or down. Here are her results. (She also calculated percentage ‘up’ and ‘down’ for each flower.)

Colour of blotches
Stamens ‘down'(%)
 

Yellow

 

57.9
 

Salmon pink

 

27.6
 

Red

 

23.8

Table 2 Class results for 144 flowers were combined and the mean percentage of flowers with stamens down was calculated.

They agreed that these results did give evidence to support their prediction. However, they also noticed that the turned down stamens in the red flowers appeared to have shed their pollen. This led to another follow-up study. A second group of pupils then predicted that ‘red flowers would have no pollen left’. To test this, they imitated a visit from a bee by pushing their little finger into different coloured flowers so that it touched the petal blotch. They then checked their finger to see if there was any pollen on it and always cleaned it before the next test. You can see in Table 3 that these results again supported the prediction. However, even though the data were less complex than that of the first group, the conclusions were not as clear-cut. Many red flowers have already shed all their pollen and very young yellow flowers have stamens that have not yet begun to shed pollen. There was also some difficulty in seeing pollen on fingers. To overcome this, some suggested using a cotton bud (home-made for economy, as a clean one is needed for each test).

Colour of blotches
Number of flowers
 

Total examined
 

Number with pollen
 

% with pollen
 

Yellow

 

41
 

23
 

56
 

Salmon pink

 

17
 

10
 

59
 

Red

 

43
 

19
 

44

Table 3 A different group of pupils tested the prediction that red flowers would have no pollen left. Here are their results. 

Of course there were limitations, but both these studies encouraged observation, led to collection of meaningful data and stimulated scientific thinking. They raised many interesting questions – leading to yet more ideas for investigations!

  • Does the number of red flowers per candle increase with time?
    [With younger pupils you might let them count the number of yellow and red flowers on a single labelled candle over a period of time]
  • Do bees visit yellow flowers more often than red ones? [Casual observations suggest they do.]
  • How long does it take for a single flower to change colour?
  • Is the change in colour affected by the length of time the flower has been exposed to sunlight? Or is some other factor involved, such as the beginning of the shedding of pollen or a visit by an insect?

[Most of the flowers examined in these studies were functionally male and therefore the arrival of pollen on the stigma would not appear to be a factor.]

It was encouraging to see how the pupils taking part in these activities became much more aware of the relationship between the structure, colour and movement of different floral parts as well as the behaviour of visiting insects. The pupils were also stimulated to take an interest in what happens to other flowers. We are grateful to staff and pupils from The Cavendish School (London NW1), and from Queensgate School (London SW7), who helped collect these data and for allowing us to publish their results in this article.

Anne Bebbington
(FSC, Juniper Hall, Surrey)

Useful references

Bebbington A (2000) Exploring a horse chestnut bud in OSMOSIS 17

Farrant P (1997) Colour in Nature Blandford, London [ISBN 0-7137-2806-X]

Hickey M, King C (1988) 100 Families of Flowering Plants CUP, 2nd edition [ISBN 0 521 33700 3]

Proctor M, Yeo P, Lack A (1996) The Natural History of Pollination Harper Collins, London [ISBN 000 219906 8]

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