Sun, Sand and Orchids

Today's Guardian Country Diary describes a recent visit to the seaside at Cleethorpes. If you walk on past the pier with all its attractions, at the far end of the promenade, beyond the leisure centre and just under the sea wall near the miniature railway, you'll reach Cleethorpes Local Nature reserve. 

It's a long expanse of sand dunes, between the sea wall and the saltmarsh on the edge of the river Humber, and it hosts a fabulous display of southern marsh orchids that were in full bloom at the end of May.

There are some magnificent specimens that....

.... are packed together so densely in places that you need to be very careful where you put your feet.

While we were there I repeated Darwin's experiment for extracting the pollinia from an orchid flower, described in his book Fertilisation of Orchids, which I described in a recent post on early purple orchids. The pollinia are whole stamens that butterflies carry away in their entirety when they poke their proboscis into a nectar spur at the rear of the individual orchid flower. Darwin described how you could extract a pollinium, which is sticky at its base, by poking a sharp pencil point into the nectar spur. The paired pollinia are located where those two dark club-shaped structures are in the hood of the flower.

I didn't have a pencil so I poked this fine twig in instead - and out it came with a pollinium attached.

Darwin actually counted and calculated the number of pollen grains in one of these pollinia: there are enough to fertilise over 120,000 seeds. The survival chances of these tiny wind-born seeds, which you can see by clicking here, are minute.

Orchids Do It Differently

Most flowers discretely dust their pollen on visiting insects, but orchids go the whole hog and glue their entire stamens to their visitors. The paired stamens – known as pollinia in orchids - sit inside the hooded upper petal (top photograph) and the nectar supply is hidden deep within the flower, so even long-tongued bees like the one in the second photograph must force their head in to reach it. And that’s when they make contact with the base of the pollinia, which become glued to their head, so when the bee leaves it carries the pollinia with it. By the time it’s reached the next plant each pollinium has split open, exposing pollen that’s dusted on to the stigma of the receiving plant. You can see the white, ruptured pollinia just below the antenna on the face of the bee in the third picture, and the same area enlarged on the final image. Bees are often irritated by having these objects glued to their head and sometimes try to comb them off, but seldom succeed. Why do orchids operate in this way? Well each individual orchid flower can produce hundreds – sometimes thousands – of ovules and it needs a comparable number of pollen grains to ensure that they’ll all be fertilised and develop into seeds – despatching a whole stamen’s worth of pollen in one go, glued to an insect, does the trick very nicely.