Hornworts lure bacteria into slimy chambers to farm nitrogen-fixing partners
Many hornworts grow internal cavities filled with mucilage, then release chemical signals that coax free-living cyanobacteria, especially Nostoc, to move in. The resulting colonies tint the little plant blue-green. It is one of several oddities that set these humble, horn-bearing plants apart from their cousins the mosses and liverworts.
Hornworts form the division Anthocerotophyta, a group of non-vascular land plants found worldwide in damp places, sometimes as tiny weeds in gardens and fields, while tropical Dendroceros species grow on tree bark. The flat green body, usually a rosette or ribbon one to five centimetres across, is the gametophyte, which carries a single set of chromosomes. The name comes from the horn-like sporophyte that rises from it.
That horn grows in an unusual way. Moss sporophytes grow from the tip and liverwort sporophytes from the middle, but a hornwort's lengthens continuously from a meristem at its base, spending its whole life attached to the parent. Unlike liverworts, most hornworts have true stomata on the sporophyte. Inside runs a central rod called the columella, surrounded by tissue that makes spores and multicellular pseudo-elaters, which twist as they dry to scatter the spores. Even the zygote behaves differently: its first division is lengthwise, unlike in any other bryophyte.
Their cells are distinctive too. Most hornworts have lost two genes involved in plastid division and keep just one chloroplast per cell, though Megaceros and some Nothoceros and Anthoceros species have several. Yellow and brown spores have thick, oily walls that let them survive for years, while a few genera produce short-lived green spores. Spores measure between 30 and 80 micrometres or more, large for the group.
Counting them is hard. More than 300 species names have been published, yet the true total may be as low as 100 to 150, and other estimates put it near 200. Hornworts are among the earliest-diverging land plant lineages, probably older than the Devonian. Crown-group fossils only begin in the upper Cretaceous, though the Devonian plant Horneophyton may be an early relative. Long treated as a class of bryophytes, they were later given their own division, and recent evidence again suggests bryophytes form one natural group.
Source: Hornwort
