The hornworts are a widespread group of nonvascular plants related to mosses and liverworts. In fact, they were once placed in the same division Bryophyta. Hornworts are not related to the family of submersed aquatic angiosperms (Ceratophyllaceae) also called hornworts. "Hornwort" refers to the slender, erect sporophyte embedded in the basal, thallus-like gametophyte. As in other bryophytes the diploid sporophyte remains attached to its parental haploid gametophyte throughout its life, but unlike other bryophytes, the sporophyte continues to grow throughout its life. Another unusual hornwort characteristic is a single large chloroplast in each cell. Fossil hornwort spores date back to the Miocene in Europe. The first intact fossil hornwort was discovered in 25 million-year-old Dominican Republic amber.

Hornwort Life Cycle Courtesy of Wikimedia Commons

The life cycle and morphology of hornworts (division Anthocerotophyta) has characteristics in common with liverworts and mosses (divisions Marchantiophyta and Bryophyta). In fact, older references reduced all three groups to classes within the division Bryophyta. New evidence from DNA sequencing and cladistic analysis reveals that they represent three distinct divisions (phyla). Unlike mosses and liverworts, the hornwort sporophyte continues to grow throughout its life from a meristematic region near the base and not from the tip as in other plants. Unlike liverworts, most hornworts have true stomata on their sporophytes as in mosses. The slender, erect sporophyte has a multicellular outer layer, a central rod-like columella, and a layer of tissue in between that produces meiospores and pseudoelaters. At maturity the sporophyte splits open just below the apex to release the spores inside. The pseudoelaters are multicellular, unlike the elaters of liverworts. Helical thickenings cause the pseudoelaters to change shape and twist as they dry out, thereby helping to disperse the mass of trilete spores. When trilete spores separate from the common tetrad, each spore shows 3 lines radiating from a central pole. In monolete spores there is a single line on the spore surface indicating that the mother cell split into 4 along a vertical axis. The earliest land plants, such as Horneophyton and Rhynia also had trilete spores.

Like liverworts, the haploid gametophyte (thallus) is dorsiventrally flattened. It often becomes slimy due to mucilage-filled cavities when groups of cells break down. These cavities are invaded by colonies of cyanobacteria (Nostoc) giving the thallus a blue-green color. One of the most unusual characteristics is the single large chloroplast per cell. A single chloroplast is common in green algae (division Chlorophyta) but it is quite uncommon and unusual for true multicellular plants (kingdom Plantae). Like mosses and liverworts the motile sperm are biflagellate and swim to the egg via water.

Cladistic analyses suggest that the Anthocerotophyta originated much earlier in the history of land plants, possibly before the Devonian. Hornworts may even be one of the earliest lineages of land plants. Some of their morphological characteristics are similar to sporophytes of the ancient land plant Horneophyton that lacked true vascular tissue. In fact, some paleobotanists have suggested that Horneophyton may be the "missing link" between hornworts and the Rhyniopsida, an extinct class of early vascular plants. Fossils of these ancient land plants are well represented in the early Devonian Rhynie Chert beds in Aberdeenshire in the north of Scotland. This time period is approximately 398-416 million years ago, long before the age of dinosaurs. Hornworts are certainly one of the most unusual and fascinating plant groups on earth.

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