Friday, 6 March 2009

Wind Pollination

Many flowering plants are insect pollinated. The flowers attract insects through visual cues, scent and nectar. Once attracted the insect picks up sticky pollen grains from the flower’s stamens (the male parts of the flower). On visiting a second flower the pollen grains are deposited on the stigma (the female part of the flower) and, hence, the flower is ‘pollinated’ (i.e. fertilised). Subsequently fruits and seeds develop.
Some plants, though, are wind pollinated. Most British tree species are wind pollinated, as are grass species and some species of flowering herb. Many wind pollinated tree species produce ‘catkins’ which are long, dangling, pollen-bearing ‘tassels’. The catkins are composed of many scale-like structures between which are the stamens. Once the catkin is fully extended it dangles in the wind, the scales open and the pollen is displaced from between them by air currents. Some of this air-borne pollen eventually encounters a female flower, which is equipped with stigmas, and pollination occurs. These female flowers are usually carried on the same tree but separately from the male catkins.
Wind pollination is immensely ‘inefficient’ in terms of pollen production. Huge amounts of pollen is produced, much of which is wasted because it fails to encounter a receptive stigma. It has been estimated that a single Hazel catkin produces nearly four million pollen grains. Pollen grains from wind pollinated plants are, proportionally, much smaller than those produced by insect pollinated plants and they tend to be smooth rather than sticky. Subtle mechanisms are in place to avoid auto-fertilisation (there is more information on this in the references).
Many wind pollinated trees produce their catkins early in the year before the trees’ leaves appear. This avoids the possibility of leaf surfaces trapping, or impeding the progress of, pollen grains through the air.
In late February to mid March two excellent examples of wind pollinated trees in the Mersey Valley are Hazel (Corylus avellana) and Common Alder (Alnus glutinosa). The top photograph shows seven Hazel catkins. The female flowers are tiny, scarlet brush-like structures and there is one near the top right of the photograph and another near the extreme left. Once pollinated these gradually develop into Hazel nuts.
The bottom photograph shows Alder catkins. This time the female flowers are small, red, bristly ovoid structures near the centre of the photograph. Once fertilised these develop into small, woody ‘cones’ (rather like miniature fir cones). Last year’s cones are still present and are visible near the top right of the photograph.
The most ‘historic’ Hazels in this area are probably those along Hawthorn Lane (the old border between Chorlton and Stretford). These have been grievously ‘hacked about’, in recent years, by the local authorities (who probably don’t know what they are, don’t understand their significance and probably wouldn’t care if they did!).
Some of the finest Alders are on the little triangle of land formed where Chorlton Brook flows into the Mersey. I am convinced that these are self-seeded. This is a reminder that Alders tend to be found along water courses because the seeds are water-proof and buoyant and float down-stream until they lodge in the bank. Alders can happily grow in water-logged ground and have nitrogen-fixing nodules on their roots. If you take a train journey, at this time of year, look out for a sort of ‘reddish haze’ above water courses - the effect of millions of Alder catkins and female flowers.

Dave Bishop, March 2009.


1. ‘The Pollination of Flowers’ by Michael Proctor & Peter Yeo, Collins 1973

2. ‘The Biology of Flowers’ by Eigil Holm & Thomas Bredsdorff, Penguin 1979

3. ‘How Flowers Work’ by Bob Gibbons, Blandford Press 1984

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