Photo Tips for Grimmiaceae

Even moreso than Bryaceae, Grimmiaceae is a moss group that really begs for microscopy to navigate the often subtle or hidden differences between species. When fertile and mature, it is usually easy to distinguish between the four common regional genera, but getting to species can be very challenging save for the taxa that are either habitat specific or particularly luxuriant in one feature or another. Before I go any further I would be remiss without mentioning the best regional resource for this group: @david1945wagner 's broadly applicable Guide to Racomitrium of Oregon website, which has a visual key, taxonomic notes, comments on variation and distribution, and lavish photomicrographs to show relevant microscopic characters for species of Racomitrium known from Oregon (among other places).

As with any series of bryophyte photos you might take to represent something you see (likely growing on rock, in the case of this family), identification will always benefit from the following general shots:

(1) Habitat shot (showing substrate and associated bryophytes)
(2) Detailed habit shot of the bryophyte showing its general growth form (cushioned, tufted, upright, scrambling, long leaves, hair points, distribution and form of sporophytes if present). Something for scale also helps here.
(3) Detail of the leaf -- best shot with transmitted light with the leaf held between yourself and the sky/light source.
(4) Detail of the sporophyte (if present) showing features like calyptrae, operculum, teeth, orientation where and when possible.

I have found it almost impossible to work with juvenile material of Grimmiaceae. Coscinodon, Schistidium, Grimmia and Racomitrium can look very similar when young. As the leaves mature and the sporophytes emerge the differences begin to demonstrate themselves. You need only photograph them at this point to tell apart the genera. With a few extra flourishes, you might be able to get closer to a species ID, though for the majority of the taxa in this family you will need a microscope to be sure. More often than not I find myself unsure, even with a microscope.

Here is what I want to see when I am trying to ID memebers of this family

(1) Notes on the rock type and habitat. Some species are limestone (eg. Schistidium duprettii) specific of only occur along shoreline rocks (eg. Schistidium maritum) or riverside boulders (eg. Schistidium rivulare). Typically, only two species grow on wood and bark (Racomitrium heterostichum and Racomitrium varium). I have never seen Racomitrium on limestone.
(2) A detailed shot of the sporophyte showing if the capsule is immersed (as in most Schistidium and some Grimmia), exserted (as in all Racomitrium, Coscinodon & most Grimmia), the seta twisted (as in Racomitrium), the peristome teeth elongate (as in some Racomitrium), colour of the teeth (cherry red in many Schistidium).
(3) If possible (usually hard with Grimmia & Coscinodon), pull a shoot off so you can photograph the branching pattern. Species like R. elongatum have regular, short pom-pom branches, while others like Schistidium papillosum and R. lawtoniae show far fewer and less regular branches.
(4) If possible, hold a cluster of leaves up to the sky and shoot a detailed transmitted light shot of the leaf apex. Most Grimmiaceae species have a clear tip, while some lack said tip and have obtuse and dentate apices. With a strong enough macro or a cell-phone+hand lens combo, you should be able to make out the presence of teeth on the margins (as in R. acicularis) and on the awn (as in R. lanuginosum). If you are really lucky, you might even be able to make out whether the hair points/awns run down the margins of the leaf ("decurrent") or come to an abrupt end along a horizontal line of demarcation between photosynthetic and non-photosynthetic cells. You should be able to see these details in the observations below of Racomitrium lanuginosum.

Linked below are example of photographs of taxa within Grimmiaceae showing types of shots that are helpful. Having said all that, this is a really hard group and in many cases you will hit a brick wall without a microscope.

Posted on February 2, 2024 03:41 AM by rambryum