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Arrangement of the leaves
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Vegetative characters
Some thoughts on bryophyte species identification...

When I first attempted to meet the challenge of moss and liverwort species identification, I tried to apply similar methods to those I use in identifying unfamiliar flowering plants, that is to learn the characters of the higher-level taxa to which each species belongs.

Familiarity with flowering plant families is a good start to homing in on potential candidate species, and I assumed the same would be true of bryophytes and their higher-level taxa. I was informed by expert bryologists that this approach would not work for bryophytes, and that I'd just have to learn the species one by one.

Experience has shown me that the experts were right, indeed the British Bryological Society Field Guide doesn't mention families at all. The text does note the order to which each described species belongs, but that doesn't narrow the options down very much. In contrast, it's hard to envisage a credible flowering plant field guide that didn't name the families for all species covered.

The key reason for this difference is, I think, that for the most part the characters being used to identify bryophytes are details of the gametophyte which are, of course, exclusively vegetative features. And as with flowering plants, the vegetative organs tend to assume morphologies dependent more on habitat than on taxonomic position. Attempting to identify flowering plants by vegetative characters alone would present similar difficulties to that encountered in determining bryophyte species.

Leaves spirally inserted, forming rosettes on a single shoot
  • Atrichum undulatum
  • Barbula sardoa
  • Barbula unguiculata
  • Bryum capillare
  • Bryum pallens
  • Dichodontium palustre
  • Funaria hygrometrica
  • Gymnostomum aeruginosum
  • Gyroweisia tenuis
  • Hedwigia stellata
  • Mnium hornum
  • Orthotrichum cupulatum
  • Orthotrichum diaphanum
  • Orthotrichum stramineum
  • Pogonatum aloides
  • Pohlia melanodon
  • Polytrichum commune
  • Polytrichum piliferum
  • Pseudocrossidium revolutum
  • Racomitrium fasciculare
  • Racomitrium heterostichum
Dichodontium palustre Dichodontium palustre (Dicranaceae)

  • Rhizomnium punctatum
  • Schistidium rivulare
  • Syntrichia laevipila
  • Syntrichia ruralis ssp. ruraliformis
  • Tetraplodon mnioides
  • Tortula muralis
  • Trichostomum brachydontium
  • Ulota bruchii
  • Ulota phyllantha
Dichodontium palustre

Spiral leaf insertion

In Dichodontium palustre the spirally diverging leaves are carried throughout the length of the central shoot. In vascular plants, different leaf insertion angles are described by phyllotaxy. Are phyllotactic terms also applicable to bryophytes?

In this category I'm including mosses with a single rosette borne at the apex of the main shoot, as well as species where spirally-arranged leaves traverse the length of the main shoot. In both cases, the main shoots present a star-shaped appearance when viewed end on.

Many erect mosses show this leaf arrangement, but it is to be found in some prostrate species too. Presumably it's more a function of inherent growth patterns arising from how the apical cell divides, rather than a particular adaptation to the environment.

  • Fissidens bryoides
  • Fissidens dubius
  • Fissidens taxifolius
Fissidens taxifolius Fissidens taxifolius (Fissidentaceae)
Fissidens taxifolius

Distichous leaves

Species of Fissidens demonstrate the distichous leaf arrangement very clearly, as in F. taxifolius shown here. Mosses with two-ranked leaves present their photosynthetic tissues efficiently with minimal self-shading.

I'm using the term distichous here to cover both opposite and alternate leaf arrangements, where two rows of leaves in the same plane are apparent on either side of a shoot.

Mosses show the three fundamental leaf arrangements: spirally inserted, arising on two opposite sides of the shoot (distichous) and arising from one side only (secund). In each case, there's presumably selection pressure tending to reduce one leaf shading another, but which arrangement is most efficient?

  • Dicranum majus
  • Palustriella commutata
Dicranum majus Dicranum majus (Dicranaceae)
Dicranum majus

Secund leaves

Dicranum majus is a large moss with leaves mostly oriented to the one side. As with all the leaf arrangement variations, this character seems to follow more taxonomic lines than habitat and niche preference.

For this character I've used a loose interpretation of secund by including species where the leaves don't necessarily arise on only one side of the shoot, but which are curved mostly to the one side.

Having most leaves oriented to one side of the plant is not necessarily inefficient in terms of utilisation of available space, as leaves on one shoot will not be shaded by the adjacent shoot. Of course, this assumes that the shoots are clumped together and share the same leaf orientation, but these features do seem frequent in secund moss species.

Leaves widely-spaced along length of shoot
  • Aulacomnium androgynum
  • Bryum pseudotriquetrum
  • Eurhynchium striatum
  • Plagiomnium ellipticum
  • Plagiomnium undulatum
  • Rhizomnium rostratum
Rhizomnium rostratum Rhizomnium rostratum (Mniaceae)
Rhizomnium rostratum

Widely-spaced leaves

This shoot of Rhizomnium rostratum shows the relatively large size of the leaves, the side margins of successive leaves not far from one another. Liverworts with similarly large leaves often have overlapping leaves.

It's arguable whether this character is a leaf arrangement as such, but it is a conspicuous character. Large, widely-spaced leaves are particularly evident in the members of the Mniaceae in the table (Plagiomnium and Rhizomnium).

Mosses with widely-spaced leaves often also have large leaves, presumably as the latter would shade one another if more closely packed. And large leaves seem most frequent in species that occupy damp and wet habitats, the luxuriant development no doubt reflecting the availability of abundant moisture.

Arrangement of the leaves lists
    The moss line
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