Ahhhh-tishooooo!

For hay fever sufferers, this can be one of the most miserable times of the year, thanks to grasses and trees that produce vast quantities of wind-borne pollen. Nevertheless, pollen grains are beautiful natural objects. The pollen grains above belong to a tropical plant called cup-and-saucer vine Cobaea scandens, which is pollinated by bats rather than by wind (do bats suffer from hay fever? Probably not...).

I stained the pollen with a fluorescent dye called acridine orange, which binds to the surface of the pollen grain and fluoresces yellow when you shine blue light on it - revealing this exquisite pattern of hexagons and pores (showing green here). Each pollen grain is about one fifth of a millimetre in diameter. Plant genera can often be identified by the distinctive pattern on their surface.

The outer surface of pollen is full of minute pits and chambers that contain proteins that allow a plant to recognise pollen of its own species when it lands on its stigma, and reject foreign pollen. It's these proteins that quite literally get up your nose, trigger an allergenic reaction and set you off sniffing and sneezing.

The outer casing (known as the exine) is made up of a polymer called sporopollenin, which is incredibly resistant to biodegradation - which is why palaeobotanists can recover ancient pollen samples from deep in peat bogs and lake beds and extract and identify pollen samples from plants that grew there tens of thousands of years ago. It's a branch of botany that has given some very useful insights into how plant species distributions have changed during periods of rapid climate change, like the one we are experiencing now: studying the past in this way gives an insight in what is likely to happen to plant species in the future.

Since the exine of pollen is so resilient, it passes through the gut of insects unharmed, although the pollen contents are digested. Yes, that little white speck on this bumblebee's tail is bee-poo, made up of empty exines of pollen that it has eaten. Many hoverflies feed almost exclusive on pollen, leaving little piles of hoverfly poo on leaves, and I know of at least one enterprising entomologist who has collected and analysed this, in order to study hoverfly's pollen diet.

During the Vietnam war Yellow Rain - yellow specks coating plants in the jungle - was believed to be the result of Communist chemical warfare. Subsequent anaysis showed that it was bee faeces, produced by vast swarms of bees that sometimes rose into the air and defecated in unison.

Welsh Poppy

Welsh poppy Meconopsis cambrica has spread all over my garden in recent years, seeding itself in all sorts of unlikely places, including the inside of my greenhouse. It’s a prolific seed producer. Its ring of stamens produces a large amount of pollen (top picture) which is a great attraction for the hoverflies that fed on this and transfer some pollen to the stigma, where it germinates. In the second and third pictures down you can see the glassy finger-like structures (papillae) that cover the stigma, where the pollen grains stick and germinate. The pollen tube that they produce penetrates these stigmatic papillae and grows downwards towards the ovules that you can see in the vertically sectioned flower in the fourth photograph. In the bottom photograph you can just make out the patterning on the glassy ovules which will develop on the seed coat, once the pollen tubes reach and fertilised them.

Foxtail’s Filters

Take a close look at meadow foxtail grass’s flower spikes in spring and you’ll find that the youngest have just begun to produce their feathery white stigmas (bottom photo), while more advanced flower spikes are releasing pollen from their dangling stamens (second up from the bottom). Under the microscope the stigmas are revealed at feathery combs of transparent cells whose job is to filter out the airborne pollen (third photo from bottom, x100). Once they’ve trapped a pollen grain it germinates, producing a pollen tube that grows down through the stigma cells (top photo x400), carrying the male cells down to the egg cells in the flower ovary, where they fuse together and begin the process of seed formation. In the top photo you can see the pollen tube emerging from the germinating spherical pollen grain and growing down through a branch of the feathery stigma. There's more on meadow foxtail grass on my other blog at http://cabinetofcuriosities-greenfingers.blogspot.com/

Blowin’ in the Wind

The male cones of Scots pine are just beginning to release their pollen now, and if you give a branch with ripe pollen sacs a sharp tap it will more or less disappear in a cloud of pollen. Conifers depend on the wind to deliver their pollen, so tend to produce vast quantities of the stuff to ensure that at least a few pollen grains make the successful journey to an ovule, fertilise it and produce a seed. The studio shot of larch pollen here (middle picture), with the yellow pollen sacs releasing pollen that’s landing on the pink young female cone, where the seeds will eventually develop, gives a false impression of the likely success rate. The chance of an individual pollen grain effecting a fertilisation is probably one in several million. The longer the pollen stays aloft, the better its chance in this lottery, so conifer pollen is slung between two balloon-like air sacs that increase its aerial buoyancy. You can see these in the top two microscope photos, at x100 and x400 magnification. They certainly seem to do the job- researchers have collected pine pollen from North American conifer forests on sticky traps mounted on weather ships in mid-Atlantic.