The green part we usually see—the gametophyte—is a flat, lobed thallus. Inside each cell lies a single large chloroplast, often with a pyrenoid, a structure rare among land plants but common in algae. This setup helps concentrate CO₂ for photosynthesis, suggesting that P. carolinianus has preserved some deep evolutionary traits from its aquatic ancestors (Villarreal & Renzaglia, 2015).
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| Phaeoceros carolinianus |
A Cyanobacterial Alliance
One of the most fascinating features of Phaeoceros carolinianus is its close relationship with cyanobacteria, especially those of the genus Nostoc. The hornwort forms mucilage-filled cavities that open to the outside via small pores. These cavities act as a home for symbiotic Nostoc, which enters through the pores and begins fixing atmospheric nitrogen into a usable form for the plant (Adams & Duggan, 2008).
This symbiosis is more than just a happy accident. P. carolinianus appears to actively recruit its microbial partner using chemical signals. Once inside, the cyanobacteria differentiate into heterocysts, specialized cells for nitrogen fixation. This allows P. carolinianus to grow in nutrient-poor soils where other plants might struggle (Adams & Duggan, 2008).
In turn, the cyanobacteria benefit from a stable environment and carbohydrates from the plant’s photosynthesis (Adams & Duggan, 2008).
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| Dark Nostoc can be seen through the thallus |
A Window into Plant Evolution
What really makes P. carolinianus a scientific treasure is its evolutionary position. Hornworts as a group may be the closest living relatives to vascular plants, more so than mosses or liverworts. That makes them especially valuable for studying the traits that emerged during the colonization of land (Villarreal & Renzaglia, 2015).
In a 2021 study, researchers examined a family of enzymes called M20D auxin-conjugate peptidases in hornworts, which help regulate the plant hormone auxin—a key player in plant growth and development. These enzymes are involved in breaking down auxin conjugates, releasing active hormone when needed. The presence of these enzymes in hornworts like P. carolinianus suggests that this system of hormonal regulation predates the rise of vascular plants (Kreitlow et al., 2021).
In other words, hornworts retain ancestral versions of growth-regulating genes that later evolved into more complex networks in ferns, conifers, and flowering plants.
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| cross section of a thallus, the large Chloroplast is visible |
Simplicity as a Superpower
What makes hornworts—and P. carolinianus in particular—so valuable for science is their genetic simplicity. With relatively small genomes and fewer gene duplications compared to vascular plants, hornworts offer a "cleaner" view of early plant evolution. Their traits are not only ancient—they’re also intact, unclouded by the evolutionary detours taken by more complex plant lineages.
And yet, they manage remarkable feats: continuous sporophyte growth, active recruitment of symbiotic nitrogen-fixers, and algal-style CO₂-concentrating mechanisms. P. carolinianus may be small, but it’s a living time capsule, showing us what early land plants might have looked and functioned like.
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| Sporophte with yellow spores |
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| Spore |
References
Kreitlow, R. L., Cuneo, P., Zhang, Y., & Clark, S. E. (2021). Examination of the M20D Auxin Conjugate Peptidase Family from Hornwort and Implications on the Evolution of the Tracheophytes. bioRxiv. https://doi.org/10.1101/2021.08.09.455726
Renzaglia, K. S., Villarreal, J. C., & Duff, R. J. (2007). New insights into morphology, anatomy, and systematics of hornworts. The Bryologist, 110(2), 214–243. https://doi.org/10.1639/0007-2745(2007)110[214:NIIMAA]2.0.CO;2
Villarreal, J. C., & Renzaglia, K. S. (2015). The hornworts: Important advances in early land plant evolution. The Bryologist, 118(4), 335–351. https://doi.org/10.1639/0007-2745-118.4.335