Hornworts, or the nature of discovery

What is discovery made of? Which are the ingredients? And how are the different ingredients combined into “finding something new”? I’ll try to give some perspectives on this, through my personal experience with a botanical discovery (which lead to my first scientific publication).

Losing the way is the way

I guess most botanists/naturalists have a “botanical bucket-list”. A list of plant rarities and oddities, of weird and wonderful plants that you wish one day to see in the field with your very own eyes. Since I tend to lean to the weird and atypical (as you might’ve guessed from this blog), my list contains some cryptogams (fern, mosses and allies).

In 2018, I went to look for Isoetes1 (an aquatic relative of ferns) in the Montnegre Natural Parc (Catalonia, NE Spain). But I took the wrong path through the mountain, and ended up going through a nice path by a slope. As I walk, I like to glance sideways at slopes, since they are very interesting places to look for bryophytes and lichens. That day, as I was walking, I noticed that the slope had dark patches, and upon getting near it, it seemed that it was covered in black hairs.

Hornworts with mature sporophytes (looking as black threads). Both pictures taken in the Montnegre National Park.

I took one of the black hairs in my hand, and I noticed that it was actually divided in two halves, opening from top to bottom. I realized that I had come across a hornwort sporophyte, and proceeded to jump with joy.

A couple of sporophytes of Anthoceros punctatus (Left), showing how they open in two halves from top to bottom. The dark colour of the mature spores gives a dark colour to the tip of the sporangium (Right).

I had learned about hornworts during my biology degree, but I had only seen drawings, pictures or half-dried specimens in the lab. That didn’t really do it for me. I wanted to see them in the field. In fact, in 2015, I did an excursion to a published hornwort locality, but unfortunately, I couldn’t find any. Funnily enough, I had gone on an excursion to see a plant that was on my “botanical bucket list” (Isoetes), and ended up finding another plant on my list (hornworts).

I love Koroks.

What are hornworts anyway?

Glad you asked! 😀

In case you aren’t familiar with hornworts, they are one of the three main living lineages of bryophytes (together with mosses and liverworts). Hornworts have remarkably low species numbers (around 200 species, compared to 9000 for liverworts and 12000 for mosses2), and low anatomical diversity compared to other modern bryophytes. The thallus consists of a green lamina (the haploid gametophyte, bearing the sexual tissues), from which horn-like sporangia emerge (diploid sporophyte which releases spores)3.

Thallus of Anthoceros punctatus, bearing immature sporophytes. Picture taken in the Montnegre Natural Park.

Unlike other bryophytes, the sporangium has no seta, and spores are continually generated by a meristem located at the base of the sporangium. The sporangium splits in two halves from top to bottom, following the maturation of the spores. Moreover, hornwort sporangia are known to bear stomata (some mosses also possess stomata). At the cellular level, bizarre features emerge, such as chloroplasts harbouring a pyrenoid, which is a carbon concentrating mechanism structure absent in all land plants but found in some algae (for instance in the unicellular Chlamydomonas sp.). Hornworts can build symbiotic partnerships with fungi, as well as with nitrogen-fixing bacteria.

Recent papers have taken a closer look at that snout hornworts, and have sequenced the hornwort genome, bringing new insights into the phylogeny of current bryophytes. Recent phylogenies suggest that hornworts are in fact the basal lineage of bryophytes, implying that the pyrenoid has been lost in liverworts and mosses, and that liverworts have lost stomata. Indeed, stomata appear to be expendable characters among bryophytes.

But what’s this got to do with the nature of discovery?

Yes, I’m getting back to that. But first let’s conclude with the hornwort story.

I sampled the hornwort and took it to the Bryology Lab at the Autonomous University of Barcelona to identify under a microscope (as I have no microscope of my own, yet). There, Llorenç Sáez (one of my PhD supervisors), game me some guidance with the ID. Thank you Llorenç! After that, there was another instance of coincidence that lead to a little discovery. Second, I ran into Montserrat Brugués in the corridor. As I told her that I had found a new Anthoceros punctatus locality, she gave some interesting details. Before the new location, there were only 3 known locations in Catalonia. Combing through herbarium records, she had found one of the citations to be incorrect. So she suggested I corrected that record in the publication. Thank you Montse! Read the published note here.

Finding the hornwort was triggered by chance (losing the way in the mountain). But the discovery would not have happened without curiosity (paying attention to plants). Moreover, at the University Lab, there was a mixture of chance (bumping into an expert in the corridor) and being surrounded by the right people (being at the bryology lab).

So if I had to summarize, I’d say that discovery has a good dose of randomness, but that it may also favour the prepared mind. Moreover, no one knows everything, so discovery can be enhanced when it is shared with other people.

Where are the species: the Wallacean shortfall

The discovery of a new record of a species already known in the region (and country, Iberian Peninsula, and Europe), might seem trivial. In a way, it is. It thrilled me, and it is a valid scientific contribution. And yet, it is difficult to foresee than anything major (or maybe even minor) will come out of it.

Nevertheless, we should take into account that the Wallacean shortfall (knowledge gaps on the distribution of taxa) is one of the main deficits that hinders our understanding of nature. This knowledge gap appears to be specially dramatic in groups that have received less attention, such as bryophytes. The point that we don’t know much about the distribution of bryophytes in Catalonia has been made before, and it is obvious if one checks the distribution maps in the Biodiversity Data Bank of Catalonia. Moreover, a recent paper has quantified the pattern of ignorance on bryophyte distribution for the whole Iberian Peninsula, showing there is much work to do to update the distribution of bryophytes in the Iberian Peninsula. I am happy to think that I’ve made a little contribution to help stem the Wallacean shortfall regarding Iberian bryophytes.

Bonus track: the storm against the hornwort

In January 2020 , Storm Gloria hit Spain, leaving 13 dead, 4 missing, and millions in material damage. Videos of the winds, waves and rivers were amazing, and quite scary.

The Gloria caused severe damage in the Montnegre Natural Park. I’ve seen only parts of it, and it is impressive. I was told by a friend that the storm had cause some slopes to collapse, including the slope where I found the population of A. punctatus. I have been back there, and I am happy to say the population persists. Not only there are additional patches that I hadn’t noticed on my first visits, but even where the slope collapsed, many patches of the hornwort go on.

Interestingly, I visited the location in a different month, so I found the hornwort in a different phenology. Instead of the black threads (sporophytes), the surface of the thalli was covered (or should I say punctuated?) by dots. I presume those were the locations of the sexual tissues. I also presume that the specific epithet “punctatus” comes from the punctuated aspect of the dorsal surface of the thallus.

A hornwort through a hand-held lens. Picture taken in the Montnegre Natural Park.

As a concluding remark, let me tell you what stimulated me to write this blog post. I wrote a scientific contribution a while ago, and this post is in a way a promotion of this short publication. So admittedly it’s a bit of self-promotion. I was also stimulated from my interest in hornworts (or in bryophytes, in a more general way). I tend to like obscure, overlooked, and odd biological groups 🙂

1 According to some botanist colleagues, Isoetes is no longer found in the location I wanted to visit, as the forest canopy has grown dense over the small ponds.

2 In case you are thinking that is a lot of species, you are right. There are more species of moss than species of pteridophytes or gymnosperms. Bryophytes are in fact a very diverse and succesful (although overlooked) group of plants.

3 Yes yes, the sporophyte of hornworts probably looks more like a hair rather than a horn (or any other thread-like object of your choosing). My guess is that the English common name translates the Latin common name: Anthocerophyta, in which “antho” means “flower”, and “cero” means “horn”.

References

Riera M (2019) Anthoceros punctatus, a new hornwort for the Montnegre massif. Butlletí de la Institució Catalana d’Història Natural 83: 93-94. DOI: 10.2436/20.1502.01.11 [in Catalan]

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