Hornworts

Phylum Anthocerotophyta

Hornworts are a group of bryophytes, and like their cousins the mosses and liverworts, they are non-vascular and spore-bearing. They derive their name from the persistent, horn-like structure (sporophyte) that produces spores. This group has no leaves; only a flattened green body (thallus). This group is enigmatic. It is sometimes considered to be most closely related to vascular plants, and shares some features with vascular plants, such as sunken gametangia. Other research studies dispute this interpretation, and consider hornworts to be most ancestral and more algae-like due to their lack of seta and single nucleus per cell.

Gametophyte (gamete-forming phase)

Vegetative features

Gametophyte is photosynthetic and dominant

Stems / leaves

Hornworts are thalloid like liverworts
There are no leafy forms
Cells have only one large chloroplast per cell (unique in plant kingdom, but common in green algae)
- Only land plant to possess pyrenoids, which generate and maintain a CO2-rich environment around the enzyme RuBisCo
Possess mucilage clefts which contain nitrogen-fixing cyanobacteria
- These are 2-celled pores which resemble stomata in sporophytes, but lack the complex pore anatomy, development, and function
- Stomata in gametophytes have only been observed in fossil plants, such as Lyonophyton †

"Roots"

Root-like rhizoids

Reproductive features

- Gametangia (i.e. antheridia/archegonia) are sunken or embedded in the thallus of the gametophyte

Sporophyte (spore-bearing phase)

Horn-like shape to spore case
Epiphytic and dependent on the gametophyte phase
Continuous spore production from basal meristem (*unique among plants*)
Pseudo-elaters found with spores, which change shape as they dry and assist in dispersing spores.
Possess a central column of sterile tissue (=columella) in capsule
- Similar to basal mosses (Sphagnum and Andrea) and also the extinct Horneophytes †