Slender stonewort  (Nitella gracilis)

Stoneworts used to be classified as members of the plant kingdom, but it is now agreed that they belong – along with other green algae – in the kingdom Protista. Put simply, the protistas are simple multi-celled or single celled organisms, descended from some of the earliest life-forms that appeared on Earth. Some of the Chlorophytes, specifically the stoneworts, are thought by scientists to have been the early ancestors of all plants. Stoneworts do indeed resemble plants, are frequently mistaken for them, and are often found as fossils. The main body of the plant consists of a series of "giant cells" up to several centimetres in length, which effectively makes up the stem, together with branches resembling leaves radiating out from 'nodes' that are made up of smaller cells. The stonewort anchors itself, not with roots like a plant, but with rhizoids, colourless, hair-like filaments. Like roots, these can absorb nutrients, but the organism can absorb through its entire surface. They live in fresh or brackish water, which is low in nutrients and many species require water that is high in calcium. Stoneworts are often encrusted with white lime deposits, giving a crusty texture (hence the name ‘stonewort’), and they often have an unpleasant smell, similar to stale garlic.