Bank Hair Cap Moss – Polytrichastrum formosum

Hairway to Heaven 

For a long time my pictures of this moss were only labelled “BIG MOSS”.  When I found it in the woods I knew I had to investigate as it was just so noticeable! Having now done some research, it turns out its genus has some really unique traits unlike any other bryophytes. This moss doesn’t settle for your average bryophyte water transport method. 

P. formosum is an erect, unbranched moss that grows scattered shoots in loose tufts. These can grow 5-10 cm tall with leaves slightly over 1 cm long. Big Moss was an accurate description! It has a beautiful, vivid dark green colour. 

Look at those gametophytes! No sporophytes were visible as I took this photo in January.

Most run of the mill mosses are ectohydric, meaning they rely on external water transport on the plant surface by capillarity. They have very thin, non-waxy cuticles that allow them to absorb water over the entire plant surface. 

Our very exciting bank hair cap is an endohydric moss, meaning water must be conducted from the base of the plant. Mosses are considered non-vascular plants, but Polytrichum have clearly differentiated water conducting tissue. Their hydromes, central cylinders of stem tissue, have cells called hydroids, which conduct that fresh, fresh water. Its a bit like the xylem in a vascular plant, but comes from a different evolutionary origin. The leptome that surrounds the hydrome is analogous to phloem. It’s moss, but not as we know it! 

That’s not where this genus’ secrets end though. Polytrichastrum have parallel photosynthetic lamellae on the upper surface of their leaves. Most mosses simply have a single plate of cells on the leaf surface; Poly. have more highly differentiated photosynthetic tissue. They’re classy like that. 

It’s considered a xeromorphic adaption to low-water environments. Moist air is trapped between the lamellae, with the low tissue-to-environment exposure ratio minimising water loss. However, enough gas is still able to be exchanged to allow for photosynthesis. Microorganisms such as parasitic fungi, tardigrades, and rotifers can call the lamellae micro-environment home. 

Another completely flawless illustration by yours truly: a cross section of a leaf of P. commune (a close relative of  P. formosum) showing parallel photosynthetic lamellae at 400x magnification. The green cells contain chloroplasts. 

P. formosum is found in lowland deciduous woodlands on acidic soils, such as the woods on the UEA campus. It can also be found in upland woodlands and moorlands, as long as they are well drained.

The healthy clump of bank hair moss I found growing on an old fallen oak tree. Notice how different the two types of moss are in colour and shape! P. formosum stands out especially due to its size. 

So why bother growing in a clump? Many mosses create more of a carpet structure in order to get as much exposure to the sunlight as possible. However, growing in clumps has its own benefits. 

Clump structures compensates for the lack of xylem, allowing for transport and retention of water as well as mechanical stability, which is essential if you want to make it as a land based plant. Leaves in moss clumps can be in direct contact with each other when water tension connecting neighbouring leaves, which increases the mechanical strength of the clump. Poly. shoots have leaves with hook like appendages which when in contact with a water droplet, can attach to nearby leaves and create a resilient, lattice like structure that is perfect for preventing the loss of water droplets. 

Biomechanical tests on P. formosum showed that small (1×1 cm) clumps were significantly stronger than larger clumps. “I’m sorry, what?” is my succinct summary of what the researchers concluded. More tests are being done to figure out how this moss has gone against many principals of modern engineering. This genus of moss is slowly becoming my favourite (yes everyone should have a favourite moss genus). 

Let’s discuss sporophytes, the diploid generation of the moss. It relies on the leafy gametophyte to provide it with water and nutrients. If you can get it for free why not take advantage? Am I right Mum and Dad? Please? Its seta is the long stem-like organ that connects the sporangium ( the main body of the sporophyte that produces the spores) to the gametophyte, and holds it aloft for better spore dispersal. 

The sporophytes are also responsible for the mosses’ latin name. The ancient greek words “polys” and “thrix” mean “many” and “hair” respectively. This is in reference to the hairy calyptras that protect the capsules containing the embryonic sporophyte.  Nifty. 

Nice green barnet of bank hair moss. 

“Hey wait a minute,” I hear you cry, “How come you’re 100% sure this is P. formosum and not P. commune Eve?” Truth be told I’m not, but then again I am not 100% sure about any aspect of my life, so there. But don’t lose all hope with me instantly…

The challenge is that both mosses have a starry appearance when viewed from above, similar shaped leaves and no hair point/awn, which is an extension of the costa beyond the tip of the leaf. The costa is not the moss version of a coffee shop, but instead is a midrib of specialised cells that run lengthwise through the leaf, a bit like the central vein of a leaf of a vascular plant.

However, P. commune favours much wetter areas, so what I found is most likely good old bank hair cap moss. If UEA got drier then it might be overtaken by P. piliferum or P. juniperinum. Furthermore, the back of a P. commune leaf is rounded, whereas I saw the V-shaped back of the leaf of P. formosum.  Bam!

” http://www.ukso.org/SoilsOfEngWales  is really quite helpful when trying ID plants. I fully recommend.” – Eve Cavey, full soil convert.