Liverworts can fire sperm-laden water 15 centimetres through the air
Liverworts are tiny, easily overlooked plants with no internal plumbing, yet some shoot jets of sperm-carrying water up to 15 centimetres, reaching female plants more than a metre away. Their fossil spores go back about 470 million years, making them among the oldest plant lineages found on land.
Around 9,000 species of liverwort exist, most of them leafy rather than the flat, ribbon-like thallose kind people usually picture. Individuals are typically 2 to 20 millimetres wide and under 10 centimetres long. Together with mosses and hornworts they form the bryophytes, plants without vascular tissue. They thrive in shady, damp places, especially in the wet tropics, though some desert species survive full sun and complete drying out. The sea and very dry, sun-blasted places are about the only habitats they avoid.
Telling them from mosses takes care. Liverworts have single-celled rootlike rhizoids, while moss rhizoids are multicellular; their leaves lack a midrib, often sit in three ranks and can be deeply lobed. Most also carry something no other land plant has: membrane-wrapped oil bodies filled with isoprenoids. For some groups, only a microscope or an experienced bryologist can settle the identification.
Their life cycle turns the familiar one inside out. The green plant you see is haploid, carrying a single set of genes, and the diploid spore-producing stage is short-lived and depends on it for food. Sperm, equipped with two flagella, must swim through a film of water to reach eggs, helped by raindrop splashes or those remarkable jets. Once fertilised, the sporophyte grows a foot, a stalk and a capsule; spring-like cells called elaters then fling spores out when the capsule bursts. Many species also clone themselves: Marchantia polymorpha makes little discs called gemmae in cups, which rain can splash up to 120 centimetres, turning it into a notorious greenhouse weed.
Their place in the family tree has shifted. Once a class within the bryophytes, then given their own division, Marchantiophyta, they now appear to sit within a single bryophyte group, most closely allied to mosses. The lineage seems to have lost the pores called stomata that its ancestors had. Fossil spores found in Argentina in 2010 date from the Middle Ordovician.
Source: Liverwort
