Manzanitas and Buzz Pollination

Today’s Glimpse from the Field is another edition of David and Doug's San Bruno Mountain Rare Plant Alert emails, from March 5th, 2016. David Nelson and Doug Allshouse are on the Mountain every Saturday in their search for interesting, rare and endemic plants - this ongoing research is the basis for their upcoming book. Together they are writing "The Natural History of San Bruno Mountain", a long-needed and detailed natural history of the Mountain. The book will be published by the California Native Plant Society.

David Nelson, aside from being a surgeon, is a devoted Mountain Watcher and amateur botanist. Doug Allshouse is an expert on San Bruno Mountain flora & fauna who has studied the Mountain for 38 years. They send out regular SBM Rare Plant Alert emails to interested persons. David can be contacted at and Doug at Contact David if you would like to be on the SBM Rare Plant Alert List.

Yesterday was a great day in the field. The rain was light (although David left his notebook and Canon in the car), and it gave everything a wonderful fresh look, feel, and smell.

This winter has brought out a wonderful explosion of manzanita blossoms, possibly more than in previous years. Here is Arctostaphylos imbricata on Powerline Ridge, from a few weeks ago:

Such profusion of blossoms got us to wondering: what was pollinating them? We saw clouds of gnats, but we could not observe any of them on the flowers. An occasional bumblebee visited the flowers, but we doubted it could get into the narrow opening in the flower. Its proboscis might get the nectar, but no pollen would brush off on its head or body. How could manzanitas be pollinated?

Here is a sectioned view of an Arctostaphylos imbricata blossom. All manzanita blossoms are very similar, and rather more complicated than the simplified illustration of a flower in our last post on February 28, 2016, although it also follows the same "formula of four": sepals on the outside, then petals, then stamens (male parts, the anthem and the filament), and in the center, the pistil (female parts, the stigma, style, and ovary).

The flowers hang inverted, with the sepals at the top. The sepals are not green, as in most flowers, but are the same color as the fused petals, which in turn form a bell-shaped housing for the reproductive parts, with a very narrow opening at the tip and access to the flower is limited.

The stamen (below) is fascinating, with two long "antlers" extending from adjacent two small holes in the anther, which is a shaped like a double balloon, all at the end of the filament, that is decorated with delicate micro-filaments at its base.

Here are two views of the stamen, showing all of these features:

However, access to the flower is limited. The opening at the tip of the flower is rather small, on the order of 1 mm. The openings into the anther itself only are about 0.3 mm.

Even the diminutive gnats would not be gain access into the flower. But how could any insect get inside the 0.3 mm openings in the anthers, access the pollen, and transfer it to the stigma?

Aaron Schusteff, our frequent companion on the Mountain, is not only an expert on plants, he is also an expert on insects, and he told us about "buzz pollination", also known as sonication. Native bees (but not introduced honey bees, Apis mellifica) grasp the flower and vibrate their indirect flight muscles. Their wings do not move, but they create vibration that is transmitted to the flower. Great force is generated, up to 30 G's. The pollen is dislodged from the interior of the anther and it flows out the openings or pores. This form of anther is called a poricidal anther, named after these pores. All manzanitas and their relatives, the Vaccinia, have poricidal anthers and rely on buzz pollination. The inverted flower allows the pollen, excited from the anther by vibration, to drift down to the stigma below, as it passes out the opening in the flower.

See this interesting video for a great demonstration of how the bee's vibration can create a cloud of pollen - here the bee's vibration is simulated with a tuning fork:

Uploaded to You Tube by Anne Leonard on 2011-07-06.

So the small opening is not a problem and the insect does not have to fit inside. It just sonicates the flower. Our guess is that the long "antlers" extending from the pores accentuate the vibration and transmit it to the poricidal anther.

See you on the Mountain, where natural surprises greet you every day!

David & Doug