Thistledown

Individual branches at high magnification. The yellow, purple and blue colours are generated by using polarised light

The single-celled side branches resemble the barbules of a feather

Fine, single-cells hairs grow from the branches and form a light, overlapping mesh

Newly-shed thistledown, with seeds attached

We’re entering the season when plants are on the move, in the guise of seeds carried in the guts of birds that have eaten fruits, or hooked into the fur of animals, or carried on the wind. Thistledown – the very evocation of lightness and aerial buoyancy – can carry seeds miles, tens of miles, maybe even hundreds of miles on the breeze. Plants may be rooted to the spot, but their propagules are some of our planet’s most mobile travellers. Last week I was standing on a hillside in Weardale over five hundred metres above sea level, watching a continuous stream of thistledown drift past, swept up by thermals from thistle seed heads in the pastures down in the valley below. Under the microscope the main branches of thistledown (in this case from spear thistle Cirsium vulgare) can be seen to be made up of multicellular arms that with long, single tubular cells branching off, like the individual barbs of a feather, to produce an overlapping mesh that is light enough to provide enough lift to carry a seed aloft on a thermal. The colours here have been generated by the use of polarised light microscopy. Double-click for larger images.