The moisture-loving insectivorous plants that I grow in my conservatory stand in dishes of water, which inevitably accumulate a layer of ‘green slime’ on the surface. But this is no ordinary green slime. Under the microscope it’s revealed as a cyanobacterium (blue-green alga) called Nostoc, identifiable by those chains of small cells with occasional much larger ones interpolated along the rows. Those large cells are called heterocysts and the contain the enzymes that convert gaseous nitrogen in the atmosphere into soluble nitrogen compounds that plants need for their growth. These cells are living fertiliser factories. They’ve been providing an essential nutrient that plants need for growth, and maintaining natural soil fertility, for millions of years. Nostoc is present in soils everywhere – even in some deserts – but occasionally, during spells of wet weather, it proliferates inside a mucilaginous matrix and forms great convoluted balls of slime (bottom photo) that can cover large areas of bog, wet grass or soil. In that state it’s incredibly slippery: step on it and you’re likely to fall flat on your back.