An Introduction to Hornworts

Anthoceros  sp.

Anthoceros sp.

When was the last time you thought about hornworts? Have you ever thought about hornworts? If you answered no, you aren’t alone. Despite their global distribution, these tiny plants receive hardly any attention and that is a shame. Hornworts (Anthocerotophyta) have been around for a very long time. In fact, it is likely that they were some of the first plants to colonize the land roughly 300 - 400 million years ago.

To be fair, hornworts aren’t known for their size. They are generally small plants, though their colonies can form impressive mats. To find them, one must try looking in and among rocks, bare patches of soil, or pretty much anywhere enough moisture builds up to supply their needs. They tend to enjoy nutrient-poor substrates but I would hesitate to say that with any certainty. No matter where you live, from the tundra to the tropics, there is probably a hornwort native to your neck of the woods.

Dendroceros  sp.

Dendroceros sp.

How many different species of hornwort there are is apparently the subject of some debate. Some authors recognize upwards of 300 species whereas others suggest the real number hangs somewhere around 150. Regardless of the exact numbers, hornworts belong to one of six genera: Anthoceros, Dendroceros, Folioceros, Megaceros, Notothylas and Phaeoceros. Fun fact, the suffix ‘ceros’ at the end of each genus is derived from the Latin word for ‘horn.’

The reason they are called hornworts is because of their reproductive structures or “sporophytes.” Similar to their moss and liverwort cousins, hornworts undergo an alternation of generations in order to reproduce sexually. The green gametophytes house the sexual organs - antheridia if they are male and archegonia if they are female. After fertilization, a sporophyte begins to grow, which will go on to produce and disseminate spores. However, the way in which the hornwort sporophyte forms is a bit different from what we see in mosses and liverworts.

Alternation of generations in hornworts.

Alternation of generations in hornworts.

Upon fertilization, the zygote begins to divide into a bulbous mass of cells affectionately referred to as "the foot.” This foot remains within the gametophyte throughout the lifetime of the hornwort, depending on the gametophyte for water and nutrients. Even more peculiar is the the fact that the growing point of the sporophyte is at the base rather than the tip. As such, the horn of each hornwort could continue to grow upwards until it is damaged in some way.

The horn itself is an amazing structure. Whereas the outside layers of tissue are merely structural, the internal tissues differentiate into two different types - spores and pseudo-elaters. Pseudo-elaters expand and contract as humidity fluctuates so as the sporophyte splits to release the spores, the pseudo-elaters dehydrate and snap like tiny spore catapults, thus aiding in their dispersal.

Megaceros flagellaris

Megaceros flagellaris

Of course, reproduction is the main goal but to get to that point, hornworts must grow and mature. How they manage to survive is incredible because it is a reminder that what are often thought of as “primitive” plants are actually far more advanced than we give them credit for. The main body of the hornwort gametophyte is a thin layer of cells that spread out to form a tiny, green mat. This is the structure you are most likely to encounter.

Inside each cell is a single chloroplast. In most hornworts, the chloroplast does not exist in isolation. Instead, it is fused with other organelles into a structure called a “pyrenoid.” The pyrenoid functions as both a center for photosynthesis and a food storage organ. This is unique as it relates to terrestrial plants but quite common in algae. Another odd fact about hornwort anatomy are the presence of tiny cavities scattered throughout their tissues. These cavities form as clusters of hornwort cells die. They then fill with a special mucilage that appears to invite colonization by nitrogen-fixing cyanobacteria. The cyanobacteria set up shop within the cavities and provides the hornwort with supplemental nitrogen in return for a place to live.

Anthoceros agrestis

Anthoceros agrestis

Cyanobacteria aren’t the only organisms to have partnered with hornworts either. Mycorrhizal fungi also enter into the picture. A study done back in 2013 actually found that a wide variety of fungi will partner with hornworts which suggests that this symbiotic relationship is much more ancient and versatile than we once thought. Fungi cluster around parts of the gametophyte that produce root-like structures called “rhizoids,” offering nutrients in return for carbohydrates.

All in all, I think it is safe to say that hornworts are remarkable little plants. Though they can sometimes be difficult to find and properly identify, they nonetheless offer plenty of inspiration for the botanically inclined mind. We can all do better by tiny plants like the hornworts. They have been on land for an incredible amount of time and they definitely deserve our respect and admiration.

Photo Credits: [1] [2] [3] [4] [5]

Further Reading: [1] [2] [3]

Tropical Ferns in Temperate North America

All plants undergo some form of alternation of generations. It is the process in which, through reproduction, they cycle between a haploid gametophyte stage and a diploid sporophyte stage. In ferns and lycophytes, this alternation of generations has been taken to the extreme. Instead of the sporophyte relying on the gametophyte for sustenance, the two generations are physically independent and thus separated from one another. In a handful of fern genera here in North America, this has led to some intriguing and, dare I say, downright puzzling distributions.

The presence of a small handful of tropical fern genera in temperate North America has generated multiple scientific investigations since the early 1900's. However, as is constantly happening in science, as soon as we answer one question, seemingly infinite more questions arise. At the very least, the presence of these ferns in temperate regions offers us a tantalizing window into North America’s ancient past.

To say any of these ferns offer the casual observer much to look at would be a bit of an exaggeration. They do not play out their lives in typical fern fashion. These out-of-place tropical ferns exists entirely as asexual colonies of gametophytes, reproducing solely by tiny bundles of cells called gemmae. What's more, you will only find them tucked away in the damp, sheltered nooks and crannies of rocky overhangs and waterfalls. Buffered by unique microclimates, it is very likely that these fern species have existed in these far away corners for a very, very long time. The last time their respective habitats approached anything resembling a tropical climate was over 60 million years ago. Some have suggested that they have been able to hang on in their reduced form for unthinkable lengths of time in these sheltered habitats. Warm, wet air gets funneled into the crevices and canyons where they grow, protecting them from the deep freezes so common in these temperate regions. Others have suggested that their spores blew in from other regions around the world and, through chance, a few landed in the right spots for the persistence of their gametophyte stages.

The type of habitat you can expect to find these gametophytes.

Aside from their mysterious origins, there is also the matter of why we never find a mature sporophyte of any of these ferns. At least 4 species in North America are known to exist this way - Grammitis nimbata, Hymenophyllum tunbridgense, Vittaria appalachiana, and a member of the genus Trichomanes, most of which are restricted to a small region of southern Appalachia. In the early 1980's, an attempt at coaxing sporophyte production from V. appalachiana was made. Researchers at the University of Tennessee brought a few batches of gametophytes into cultivation. In the confines of the lab, under strictly controlled conditions, they were able to convince some of the gametophytes to produce sporophytes. As these tiny sporophytes developed, they were afforded a brief look at what this fern was all about. It confirmed earlier suspicions that it was indeed a member of the genus Vittaria, or as they are commonly known, the shoestring ferns. The closest living relative of this genus can be found growing in Florida, which hints at a more localized source for these odd gametophytes. However, both physiology and subsequent genetic analyses have revealed the Appalachian Vittaria to be a distinct species of its own. Thus, the mystery of its origin remains elusive.

In order to see them for yourself, you have to be willing to cram yourself into some interesting situations. They really put the emphasis on the "micro" part of the microclimate phenomenon. Also, you really have to know what you are looking for. Finding gametophytes is rarely an easy task and when you consider the myriad other bryophytes and ferns they share their sheltered habitats with, picking them out of a lineup gets all the more tricky. Your best bet is to find someone that knows exactly where they are. Once you see them for the first time, locating other populations gets a bit easier. The casual observer may not understand the resulting excitement but once you know what you are looking at, it is kind of hard not to get some goosebumps. These gametophyte colonies are a truly bizarre and wonderful component of North American flora.

Photo Credit: [1] [2]

Further Reading: [1] [2] [3] [4]