Melastoma malabathricum and buzz pollination

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Singapore Rhododendron or Sendudok (Melastome malabathricum) is commonly seen throughout the tropics. The plant is fast growing, producing attractive, pinkish mauve flowers with two types of stamens. There are five short stamens with yellow filaments (stalks) and anthers (pollen sacs) and five long stamens. The long stamens consist of two parts – the basal part is straight and yellow and the upper part curved and mauve. There are two short horns at the junction where the two parts join. The anthers are found at the ends of the curved filaments. Arising from the centre of the flower is the long pinkish style that ends in a green stigma – see image above.

Note that all ten stamens have pores at the tips of their anthers (above, long stamens; below, short stamens). These pores are where the pollen grains shoot out, triggered by the high frequency vibrations of the bee’s wings, onto the bee’s body. This can be demonstrated with a tuning fork of the right pitch when placed next to the stamens.

The large Carpenter Bees as well as smaller bees (this needs confirmation) collect pollen this way, known as buzz pollination LINK.

Unlike anthers of Melastoma malabathricum, those of most other plants rupture along defined lines to expose the pollen grains for bees, ants and other pollinators to collect (see above and below). The images above, left showing the stamens of MacArthur Palm (Ptychospermum macarthuri) and right those of Sweet Basil (Ocimun basilicum), the pollen are the many blobs of white found on the pollen sacs.

The Halictid Bee or Sweat Bee (Anthophora zonata, Family: Halictidae), whose body is black with broad bluish-green metallic banding (above), can be seen gripping the tips of the long as well as the short stamens to get at the pollen (see video below). At times, these bees grab at the stigma of the style, mistaking it for the tip of the stamen. By doing this, the bees transfer pollen, either taken from the same flower or from different flowers and in the process indulge in self or cross pollination respectively.

YC Wee
Singapore
November 2014

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