Turn over a fern frond at this time of year and you’ll find that the underside is covered in rows of what look like small blisters (bottom photo). These are scores of spore- producing structures called sporangia, clustered together under a membrane that keeps them moist while they’re developing. In the example illustrated here the membrane is kidney-shaped, which is characteristic of a buckler ferns in the genus Dryopteris. Once the spores in the sporangia are ripe the protective membrane withers and at this stage the sporangia – sometimes more than a hundred in each cluster – look like minute black eggs when you look at them with a hand lens (second and third pictures from bottom - double-click any image for an enlarged view). Each individual sporangium is a minute catapult that fires its spores out into the airstream. You need to look at a sporangium under a microscope to see its detailed structure and decipher how it works (top photo). Each sporangium, mounted in a stalk and stuffed full of spores, is egg-shaped and has a 'spinal column' of thick-walled cells (showing up in vivid colours in these polarised light micrographs) stretching about two thirds of the way around its vertical circumference. Once these cells are exposed to air they lose water through their thin outer wall that you can see in the second photograph from the top, drawing this outer wall inwards via the surface tension of the remaining water inside. This creates tension inside each cell and, repeated all the way along that ‘spinal column’ of cells, draws the 'spinal column' back like a bowstring, ripping open the sporangium and exposing the spores inside. Eventually the remaining water in each cell in the 'spinal colum', under immense surface tension, vapourises instantaneously and the natural springiness of the thick walls of the sporangium flicks the spine back to its original position, hurling out the spores like rocks from a Roman siege catapult. Once the spores - each around a hundredth of a millimetre in diameter - reach the airstream they can be carried vast distances. Ferns are often amongst the first plants to establish themselves on new volcanic islands and lava flows, thanks to their spores' incredible aerial mobility, which also allows them to colonise unlikely places in urban environments. Next time you're walking through any city, look up at the gutters and you'll see ferns growing that arrived as wind-blown spores. What happens next, after a spore lands and germinates, is an equally remarkable tale of frantic sexual reproduction .... but that’s another story.