The mechanics of pollination

HGA newsletter, June 2004

Bryan Thomas recently attended two lectures given by Professor Karl Niklas who is a visiting Erskine fellow at the University of Canterbury. He is the Liberty Hyde Bailey Professor of Plant Biology at Cornell University. The subjects of his lectures were “Allometry of plant growth” and “The Biomechanics of Wind Pollination”.

The Biometrics of Wind Pollination

In this lecture Professor Niklas demonstrated his work on airflow around female wind pollinated flower structures and how the shape influences the airflow, and the consequences for successful pollination. Using pine trees as the prime example he carried out wind tunnel tests on receptive pinecones and plotted the course of both neutrally buoyant bubbles and actual pollen grains around the cone. The cones were surprisingly efficient at trapping pollen grains in eddies circulating around and through the cones and that, by placing the actual flower deep within the cone in an apparently inaccessible position, the chance of a pollen grain landing in the right place was enhanced.

Professor Niklas observed that for wind pollinated plants the ratio of pollen to flowers was many millions to one, whereas in insect pollinated plants the ratio can be as low as a few thousand to one. 

A field study showed that pine species were more efficient at trapping pollen grains of their own species than those of other species of pine pollen.

Of interest to hazel growers was an example of a southern North American plant whose name I was unable to remember, but which physically resembled the hazel especially in respect of the tiny female flower being at the tip of a bud closely attached to the side of a small branch. Wind tunnel tests showed surprisingly that the eddies formed on the downwind side of the bud were capable of trapping pollen grains for a long period during which the pollen would repeatedly rise up the downwind side of the bud towards the tip, where it would often be blown a short distance away, only to be drawn back to the base of the bud and rise toward the tip again. Professor Niklas observed pollen grains going through many such cycles before either alighting on the flower or being blown away.

The implication for hazels and perhaps walnuts also is that it is likely that pollination is generally more successful when there is a significant wind blowing than when the air movement is very light, although the work was not able to predict optimum wind speed for successful pollination of a flower. Professor Niklas speculated that the optimum wind speed might be related to the wind speed necessary to extract pollen from the male flower, but he has not done any work on this topic.