Raindrops keep falling on my flowerhead

Imagine you’re a young and eager pol­len. You’re eager for a hot date with an ovule, but first you have to get to her door­step. How do you travel? The pop­u­lar way is to fly and many pol­len grains are light enough to get lif­ted on the breeze as the nos­trils of the world’s hay­fever suf­fer­ers know well. Heavier pol­len hitches a lift from an insect or, in more styl­ish cases, a bird. Another route is through the water. Zostera mar­ina is a mar­ine grass that is rarely exposed above the water sur­face, so it’s no sur­prise it’s evolved sub­mar­ine pol­lin­a­tion. A Chinese-Canadian team has found another aquatic route, but this in a plant you might not expect. It’s one of Charles Darwin’s favour­ite land-based plants, an orchid.

Acampe rigida comes from south-west China, but it would look great in my garden because A. rigida does some­thing very pecu­liar. It flowers when it rains.

Acampe rigida

Acampe rigida is ready for the rain. Photo by Badlydrawnboy22 / Wikimedia Commons.

The flower­ing sea­son coin­cides with the rainy sea­son in south-west China. This is odd time to aim to flower. Normally plants like to flower when insects are around, and these aren’t so com­mon when it rains. What’s going on?

Something that could be hap­pen­ing is that the orch­ids are using a weird insect for pol­lin­a­tion. So the team set up to watch the orch­ids as they flowered and see who vis­ited. They did this in 2009 and 2010. The res­ult was in 2009 there was one vis­itor to flower, a wasp. In 2010 there was noth­ing. Whatever was pol­lin­at­ing the flowers, it wasn’t insects, so what else is around dur­ing the rainy season?


The team asked if the rain­drops were pol­lin­at­ing the flowers and set decided to run a test. What they did was simple and eleg­ant. They put a shel­ter over some orch­ids and com­pared the pol­lin­a­tion of the sheltered and exposed flowers. There are some prob­lems with this. A simple umbrella won’t work because it’ll shade the plant from sun­light. You also need to allow for wind trans­fer. Despite this they set up trans­par­ent shel­ters and found that the orch­ids hit by the rain were much more suc­cess­ful with pollination.

More exper­i­ments with a shower head to sim­u­late rain revealed what was happening.

Raindrops hit the anther cap, knock­ing it out of the way and expos­ing the pol­len. They’re bounced up and then pulled back down on to the stig­mas. Contact, and self-pollination hap­pens. Effectively the rain­drops are twanging the pol­len onto the stig­mas. The team have a video of this hap­pen­ing on their blog.

This is strange stuff. In their paper Rain pol­lin­a­tion provides repro­duct­ive assur­ance in a decept­ive orchid Fan et al. note that most research shows that you don’t want repro­duct­ive organs exposed to rain. It’s poten­tial dam­age to equip­ment that you need work­ing to entice insects. Nonetheless Fan et al. have a mech­an­ism and the res­ults to show that rain has a pos­it­ive effect.

The paper is also a demon­stra­tion of the long reach of research. The old­est cita­tion in the paper is Sprengel’s work on pol­lin­a­tion from 1793. Darwin also gets name checked. It’s per­fectly pos­sible that Fan, Barrett, Lin, Chen, Zhou and Gao will be name checked in a couple of cen­tur­ies’ time in fur­ther research on aquatic pol­lin­a­tion. It puts the Impact Factor into perspective.


Fan X.L., Barrett S.C.H., Lin H., Chen L.L., Zhou X. & Gao J.Y. (2012). Rain pol­lin­a­tion provides repro­duct­ive assur­ance in a decept­ive orchid, Annals of Botany, 110 (5) 953–958. DOI:

Image: Acampe rigida by Badlydrawnboy22 / Wikimedia Commons. This image licensed under a Creative Commons by-sa licence.

Alun Salt. ORCID 0000-0002-1261-4283

When he's not the web developer for AoB Blog, Alun Salt researches something that could be mistaken for the archaeology of science. His current research is about whether there's such a thing as scientific heritage and if there is how would you recognise it?