Neil Bell

Oct 212016

Bryum dixonii growing at its type locality. Although known from other sites in Scotland this is probably the first time it has been seen here since 1898 (a few shoots on the right are a different species, Anomobryum julaceum). Photo by David Long, the first to spot it (accompanied by Gordon Rothero, David Bell and the author).

Despite its internationally important bryophyte flora Scotland has relatively few truly endemic species (perhaps four), and even some of these have a rather ambiguous taxonomic status due to their uncertain affinities to more widespread taxa. Molecular data can be critical for confirming or rejecting species as distinct and can also sometimes uncover “cryptic” species that hadn’t previously been recognised (even cryptic endemics such as the Northern Prongwort, Herbertus borealis, recently  shown to be restricted to Scotland and distinct from the Viking Prongwort, Herbertus norenus). But what if one taxonomic species turns out to be two or more distinct biological entities, and the most reliable way to tell them apart (at least initially) is their DNA? If it ever became necessary to “split” the species which entity would keep the original name and which would be “new”? The type material is the arbiter in these cases of course – the individual specimen(s) designated as the reference point for the species by the person who first described it. This is just one reason why herbaria continue to play a critical role in modern biology, in a sense underpinning the whole edifice. Without a way of knowing exactly what a scientist was referring to when he or she used a particular name, you can’t know where in the stupendously and subtly complicated diversity of the natural world to look for that novel antimalarial drug or disease resistant crop.

The formidable Reverend Hugh Neville Dixon: prolific bryologist, preacher, schoolmaster for the deaf, hockey player and 50-mile-a-day walker. Author of the celebrated “Student’s Handbook of British Mosses” (amongst many other bryological publications and, less famously perhaps, “Fen Skating”).

But type specimens are often too old to easily extract DNA from, or, given their importance, too small and delicate to risk trying. In such cases the best thing to do is to is often to go back to the precise spot where the original specimen was collected (the type locality) and collect it again. There’s a very good chance that this “topotype” material will represent the same entity as the original collection, or “holotype”.

On the 22nd of July 1898 H. N. Dixon, one of our most charismatic British bryologists, found an odd looking little Bryum in the bed of a stream on Ben Narnain in the Southern Highlands. This was later described by the French bryologist Jules Cardot as Bryum dixonii, and the species has mostly been considered to be endemic to Scotland ever since. It has subsequently been collected at a number of scattered localities across the Highlands but has always been rare. However, the picture emerging from the molecular data is suggesting a potentially complex (and as yet not fully resolved) taxonomic situation, so it’s critical to have DNA from a specimen that we can be sure is the “real” Bryum dixonii. As there had been no collections from the type locality since Dixon’s time, we determined to retrace his steps to see if his rather inconspicuous little moss was still there…


The author pointing to one of a number of clumps of Bryum dixonii occurring on these periodically irrigated mica schist slabs. The first sighting (since Dixon’s in 1898) was by David Long on the rocks further down just above the overhang, where Pohlia scotica also occurs (photo by David Long).

And indeed it was – after some initial concerns that the habitat might have been altered during the last 120 years by vegetation changes and a water intake, we found a number of colonies of Bryum dixonii in scattered crevices on flushed mica schist slabs. As an unexpected bonus it was growing in close proximity to Pohlia scotica, another putative (and actually considerably rarer) Scottish endemic moss not previously recorded from the site. This record now represents the southernmost known location globally for Pohlia scotica. Truly a significant place for Scottish bryology!

Sep 122016
Photo 11-09-2016, 12 14 22

The tiny liverwort Colura calyptrifolia (photographed with an iPhone and a x20 handlens!)

Colura calyptrifolia (or to give it its appropriately creepy-sounding common name, the Fingered Cowlwort), is one of our most fascinating UK liverworts. Absolutely tiny (the leaves are about a millimetre long and whole plants often only 2-3 mm), it is heavily modified from the basic leafy-liverwort body plan, the leaves formed into inflated sacs like miniscule balloons with pointed “beaks” at one end. These tiny sacs have even tinier trapdoor-like flaps that only open inwards, allowing them to capture ciliate protozoa and other microscopic creatures (conclusively observed in another species of the same genus). It’s not yet certain if the liverwort gains nutrients from “swallowing” these animals, although this might be a reasonable hypothesis given the similarity of the mechanism to that found in much larger carnivorous plants such as bladderworts.

Photo 11-09-2016, 12 14 20

The leaves of Colura are modified into tiny ballon-like sacs that trap small animals

This colony was spotted yesterday in Anglezarke, Lancashire following the British Bryological Society (BBS) AGM in Manchester. Populations of Colura have undergone a spectacular expansion over the last 10 or 20 years, particularly in conifer plantations where they occur as tiny epiphytes. Previously the plant was rather rare, occurring mostly on rock in humid gullies and restricted to the wetter areas of the far west. This sighting was on the trunk of a willow at the edge of a reservoir.

Something else that has changed rapidly over the last 10 or 20 years is the ease with which small things can be photographed and shared. These pictures were taken simply by pointing the camera of an iPhone through a x20 handlens (the latter much cheaper than an iPhone and even more useful!),  and if we had wished could have been made available online instantaneously. As poorly-known biodiversity is increasingly threatened globally, should we be making better use of cheap imaging and real-time networking of expertise to facilitate species discovery and monitoring?



Barthlott, W., Fischer, E., Frahm, J.-P. & Seine, R. (2000). First Experimental Evidence for Zoophagy in the Hepatic Colura. Plant Biology 2(1):93-97. 

Blockeel, T L, Bosanquet, S D S, Hill, M O and Preston, C D (2014). Atlas of British & Irish Bryophytes. Pisces Publications, Newbury.

Nov 032015

Distributions of Delongia cavallii (circles), which occurs in the East African Rift Mountains and on Réunion, and D. glacialis (squares) which spans the Himalaya from Pakistan to Yunnan

The relative structural simplicity of some groups of mosses can disguise their uniqueness, especially when simplified features have evolved multiple times within the same family from ancestors with more complex morphologies. The family Polytrichaceae is particularly well-known for very robust mosses such as the familiar Polytrichum commune, in which the leaves have sheathing bases and densely packed longitudinal “walls” of cells (lamellae) on their upper surfaces to increase the area available for the uptake of carbon dioxide and so the rate of photosynthesis in the exposed habitats they tend to grow in. This “pseudomesophyll” gives them an opaque appearance superficially similar to the leaves of vascular plants (despite the very different structure), and unlike the translucent, single-cell-layered leaves of most mosses. However, within the family a number of lineages have reverted to a smaller, more typically moss-like growth form with simpler leaves and much reduced lamellae, and it can often be difficult to know whether these share a common ancestor with such features or have developed them independently.

David Long on fieldwork in Nepal (photo by David Knott)

Piton des Neiges on Réunion, where Delongia cavallii (then Oligotrichum cavallii) was found in 2011, considerably extending its range beyond mainland East Africa (photo by Terry Hedderson)

Slopes above Yume Samdong in North-east Sikkim, a locality for Delongia glacialis. The large plant in the foreground is Rheum nobile, the Sikkim rhubarb (photo by David Long)

We have known for a few years that the smaller, morphologically simplified Polytrichaceae found in southern temperate parts of the world are not at all closely related to the similar looking species in Asian, north temperate and arctic regions placed in the genera Oligotrichum and Psilopilum. But even within Asia and the tropics it appears that there were two or three independent origins of this type of plant. In a paper just published in the journal TAXON we describe a new genus, Delongia, in honour of the recently officially retired (but still very active!) RBGE bryologist David Long, which includes two species previously placed in Oligotrichum. The name is particularly appropriate as most of the known collections of one of the species, Delongia glacialis, were made by David during his expeditions to Nepal, Sikkim and Yunnan and were first identified as Oligotrichum glaciale by Isuru Kariyawasam, an MSc student at RBGE. Interestingly, the second species, Delongia cavallii, has a quite different distribution, occurring in the East African Rift mountains and recently discovered by Terry Hedderson on the Island of Réunion.

Although molecular data were instrumental in recognising the distinctness of the new genus and its relatively distant relationship to Oligotrichum, it is also united by a number of unique anatomical features, not least spore capsules with a peculiar “spongy” lower part in which the stomata are often hidden when the capsule is dry and exposed when wet. In fact, this spongy texture had been noticed seventy years ago in O. cavallii and the rather euphonic generic name “Spoggodera” (“sponge neck” in Greek!) proposed, although this was never validly published.

A spore capsule of Delongia glacialis showing the "spongy" neck. Although the stomata are fairly exposed in this picture they become much more sunken when the capsule is dry

A spore capsule of Delongia glacialis showing the “spongy” neck. Although the stomata are fairly exposed in this picture they become sunken in distinct pits when the capsule is dry

We were able to use a type of “molecular clock” dating technique to estimate when the lineages of the two fairly different species of Delongia became separated from each other, as well as when Delongia separated from its most closely related extant genus (either Psilopilum or Atrichum s.l., not Oligotrichum). It seems that the most likely date for the separation of the Himalayan D. glacialis from the East African D. cavallii was about 23 million years ago, right at the boundary between the Oligocene and Miocene epochs and just at the time that the East African rift system was beginning to form (the Himalaya were also continuing to rise rapidly at this time).

The lineage of Delongia itself probably originated way back in the Eocene (56–34 million years ago), highlighting the importance of recognising and naming such unique and relatively evolutionarily isolated components of biological diversity that might otherwise be mistaken for recently evolved species with many close relatives. So although Delongia only has two species it certainly deserves its generic status. The value of taxonomy is that while experts might not need names to recognise the evolutionary diversity in their own specialist groups, it is only by representing this diversity in a universal nomenclature that it becomes generally accessible and quantifiable.

A chronogram (a tree with branch lengths representing absolute time) showing the likely origin of Delongia in the Eocene and the separation of the lineages of the two species around the Oligocene Miocene boundary

Bell, N.E., Kariyawasam, I., Hedderson, T.A. & Hyvönen, J. 2015. Delongia gen. nov., a new genus of Polytrichaceae (Bryophyta) with two disjunct species in East Africa and the Himalaya. Taxon 64: 893-910.

Apr 172015

The tiny tricolour axillary hairs of the moss Leptobryum wilsonii subtly advertise its true relationships

What sorts of features provide the best clues about whether or not two plants are closely related? Sometimes it’s obvious – most people can correctly recognise a daffodil (Narcissus) by its general appearance, even if individual species vary a lot in shape, size and colour. That’s because the combination of six coloured sepals and a trumpet-like corona is pretty much unique to daffodils, and all daffodils are like this because they are closely related. Often it’s less obvious however. Non-botanists might think that what we call the sycamore in the UK (Acer pseudoplatanus) must be closely related to the London plane (Platanus × acerifolia) because of its very similarly shaped leaves. In fact it isn’t at all, and both trees are much closer to other species that have completely different leaves. Leaf shape in trees is often not a very good clue to relationship, because the same forms have evolved independently many times in different groups.


Leaf cells of Leptobryum wilsonii. Size, shape and other properties of leaf cells provide important characters for identifying mosses

It’s the same with mosses of course, except that the features we have to rely on are fewer, smaller, and often less complex. A couple of weeks ago, an American colleague sent me a packet containing a few stems of a moss he had collected on a remote island south of Tierra del Fuego. “I think this could be a new genus of Rhizogoniaceae”, he said. I had studied this family for my PhD, and certainly there was some undefinable gestalt quality this moss had that reminded me of these plants. Also the cell shape, while not typical for the family, was very close to that of one particular species in the group. However, it also looked rather like a Pohlia, a rather non-descript genus with spearhead-shaped leaves. Unfortunately this plant didn’t have many truly distinctive features – except one.

The weirdest thing about this moss was its axillary hairs. Axillary hairs are minute structures produced next to the leaves of mosses. They are just a few cells long and often only visible on the youngest parts of moss stems after the leaves have been removed. It’s thought that they may have a function in protecting the immature leaves and shoots when they are themselves just collections of a few cells. As the fully mature leaves of this moss are only a little over a millimetre long, you can imagine how small the axillary hairs are! Yet to a bryological eye, these particular axillary hairs are as ostentatious as a peacock’s tail feathers.


Axillary hairs are found in most mosses, just above the leaf insertion (i.e. in the leaf axil). They are composed of just a few cells and may secrete mucilage to protect the delicate tips of immature leaves or shoots

They had an unusual “tricolour” appearance with a small orange basal cell, surmounted by a larger bright pinkish-red cell, and then a really long, club-shaped transparent cell. What’s more, they were prominent and persistent, not falling off on the older parts of the stem as is often the case. While it was the brightly and differently coloured basal cells that really drew our attention to these structures, the single long, club-shaped top cell was the biggest clue to relationships.

This axillary hair morphology pointed towards a family called the Meesiaceae, a small group that includes the very common moss Leptobryum pyriforme, a species with very long, narrow leaves that is often found as a weed in glasshouses and in flowerpots. Our moss had completely different leaves – shorter, broader and spearhead-shaped. Nonetheless it transpired that it was Leptobryum wilsonii, now thought to be the only other species of Leptobryum, much less common than Leptobryum pyriforme and restricted to South America, South Africa and Antarctica. It was not an undescribed Rhizogoniaceae, as some aspects of its habit implied, nor a Pohlia, as the leaf form suggested. However, Leptobryum wilsonii is so unlike Leptobryum pyriforme superficially, and so much more like the unrelated Pohlia, that as recently as 2008 it was named Pohlia wilsonii and is still treated under that alternativename by some bryologists (in spite of its axillary hairs!).


Misleadingly, the spearhead-shaped leaves of Leptobryum wilsonii are very similar to those of the genus Pohlia

So when some features point towards one relationship and others towards a completely different affinity, how do we decide where the truth lies? The answer is like a more analytical version of how we tell if people are telling truth – if a particular feature is consistent  with all the other information we have available and with the deep structure of that information, then we tend to have more confidence in it. However, if the consequences are that we would need to assume that a lot of other seemingly improbable things are also true (as when someone tells us that they have seen a ghost), then we are more sceptical. Sometimes we also simply trust some morphological characters more than others, just as we might trust some people more than others. We know that some features (such as leaf shape) often “lie” because they are prone to convergence in evolution, and so are more wary of them as guides to relationship.

The more features we have available as potential clues to relationship the better, because it makes it more likely that we will find a convincing consensus among the contradictory babble. That’s one reason why information from DNA sequences is now so important in taxonomy. It can provide a large enough quantity of unambiguous characters for us to find a statistically convincing consensus about one possible relationship over another.

As it happens, just recently some Japanese researchers were puzzled by a moss found submerged in lakes in coastal Antarctica. This moss was one of two species forming bizarre tower-like structures called “moss pillars” on the bottoms of these lakes.


“Moss pillars” in a lake in coastal East Anarctica (photograph courtesy of Satoshi Imura). The dominant species was identified as an aquatic form of Leptobryum wilsonii.

Like many mosses that occur as aquatics as well as on land, it had a strongly modified aquatic growth form that made it particularly difficult to identify. So they obtained DNA sequences from a number of samples, and were able to demonstrate that their aquatic plant was a form of Leptobryum wilsonii (Nakai et al. 2012, Kato et al. 2013). Furthermore, this data was also able to show very convincingly that Leptobryum wilsonii (whether in its aquatic or terrestrial forms) is indeed closely related to the familiar and very different Leptobryum pyriforme, and not at all to Pohlia. So leaf shape and the general habit of the plant are red herrings – features that have evolved independently in distantly related groups. For the plant we were attempting to identify those distinctive axillary hairs, despite their relative inconspicuousness, had been the best visible clue to its true relationships all along, pointing towards the surprising and incongruous affinity between the scruffy mosses in flower pots and the ones forming those unique “moss pillar” ecosystems in Antarctic lakes!

Nakai, R., Abe, T., Baba, T., Imura, S., Kagoshima, H., Kanda, H., Kohara, Y., Koi, A., Niki, H., Yanagihara, K., Naganuma, T. 2012. Eukaryotic phylotypes in aquatic moss pillars inhabiting a freshwater lake in East Antarctica, based on 18S rRNA gene analysis. Polar Biol. 35: 1495-1504.

Kato, K., Arikawa, T., Imura, S., Kanda, H. 2013. Molecular identification and phylogeny of an aquatic moss species in Antarctic lakes. Polar Biol. 36: 1557-1568.