- This page is about the nonvascular plant. For the aquatic plants known as hornwort: see Ceratophyllum
Hornworts are any of the species of non-vascular plants similar to moss. They were formerly classified with moss in the taxonomic phylum Bryophyta, but are now viewed as a distinct group and have been assigned to the phylum Anthocerotophyta. The phylum Anthocerotophyta is often overlooked in the plant kingdom because of their small size, despite being very common in many landscapes. This flowerless plant has about 300 known species and can be found over most temperate and tropical climates. It has two forms, the gametophyte and the sporophyte, which grows on top of and relies heavily on the gametophyte. They are thought to be the some of the first plants to move to land by evolutionists.  Hornworts are often found in moist environments. Their gametophytes are flat on the ground and usually 1 to 2 centimeters wide. The sporophytes, however, can grow up to 12 centimeters in the air.
Hornworts' gametophyte generation is a flat, irregularly shaped thallus generally less than 1.5 inches in diameter that resembles a bumpy leaf flat on the ground. The sexual organs are located on the top of the thallus. Rhizoids on the bottom of the thallus secure it to the ground.  Cells in hornworts usually have a single chloroplast. 
The sporophyte generation forms a pointed cylinder on top of the thallus. It needs the thallus to supply it with carbohydrates and moisture. New cells grow upward from the bottom of the sporophyte touching the gametophyte. Though it is photosynthetic, the sporophyte mainly functions as a container for the spores, which it eventually releases as it splits. 
Hornwort life cycles take place in an alteration of generations, which means they alternate between a gametophyte generation and a sporophyte generation.
The gametophyte generation creates the plant body that is recognizable. The male antheridium, which produces waterborne sperm, and the female archegonium, which houses the egg are located at the top of the thallus(the gametophyte plant body). Whether or not they are located on the same plant is determined by species.  The sperm and egg from the gametophyte generation create the sporophyte generation.
A single hornwort thallus can produce many sporophytes. Sporophytes are typically straight, column-like structures whose main purpose is to house the spores that will be the next generation of gametophytes. The sporophytes continue growing, adding new cells at the bottom of the plant until conditions inhibit growth fully. The sporophytes' growth is almost fully dependent on the gametophyte, as it relies on the gametophyte for many carbohydrates.  The sporophyte eventually splits open and scatters the spores. Vegetative reproduction can also occur if the thallus is seperated.
Hornworts grow best in tropical or temperate regions of the Earth, but do not do as well in cold areas. The genus Anthoceros grows all around the world, however.  Hornworts are also nonvascular plants, which means they do not have vascular tissues. Their cells must distribute water by diffusion only. This limits the areas that hornworts can live. They need moist habitats with steady, plentiful water sources. Hornworts usually live on warm rocks or damp soils in the temperate zones and tropics.  Fossils of Anthocerotophyte plants and spores have been found in Europe and other areas. 
Many evolutionists claim that non-vascular plants such as mosses, hornworts and liverworts are incredibly important in the evolutionary tree, as they were the first plants to migrate from the sea to the land. They supposedly evolved from algae and moved onto the land. However, there is a large flaw in this theory. Hornworts appear much higher in the rock layers or "earlier in the geologic timeline" than they should be if they were some of the first land plants. No tissue, or even spores for that matter, are found fossilized in the predicted layers. 
Symbiosis with Cyanobacteria
Hornworts have mucilage-filled cavities that may eventually dry out. Mucilage is a thick mucus-like viscous substance produced by many plants and other organisms. As the cavities break and are exposed to air in prokaryotic Cyanobacteria (also known as blue-green algae) take up residence and form colonies inside these now-dry mucilage cavities. These bacteria, of the Nostoc genus, are photosynthetic and contain the pigments chlorophyll a, blue phycocyanin, and red phycoerythrin.  The colonies in mucilage cavities appear as blue-green or black dots on the thallus of the sporophyte of the Hornwort. They form a symbiotic relationship with the Hornwort through nitrogen fixation. The bacteria give the Hornwort the nitrogen they have fixated, and the Hornwort gives the bacteria carbohydrates to survive on. 
Matt VonKonrat, a curator at the Field Museum of Natural History in Chicago, explains why the study of bryophytes is so important to the field of botany.
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- Speer, Brian R. [http://www.ucmp.berkeley.edu/plants/anthocerotophyta.html Introduction to the Anthocerotophyta] UCMP Berkeley. Web. Published November 26, 1995.
- The Editors of Encyclopædia Britannica Nostoc Encyclopedia Britannica. Web. Accessed January 31, 2018.