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.
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@bradenjudson @fmcghee @astorey_botany @johndreynolds @bstarzomski @dbltucker @chrisfluit @kem_luther @roanan_d @gwark @rolandwirth another entry in the "what to shoot" category. If any of you have things to add, I can incorporate them.
Great clarity - I hope to produce better material for you to work with in the future, thank you Randal!
A great topic!
Some macroscopic (mesoscopic?) characters I find useful:
Gemmae. Species like Grimmia muehlenbeckii occasionally produce gemmae in the leaf axils.
Flagelliform innovations. Grimmia funalis often has flagelliform innovations that can be seen using a hand lens, but typically require you to pick up half the colony to examine the bases of the mature shoots.
Orientation of the leaves when dry. For example, the leaves of Grimmia funalis are twisted around the stem when dry. Similarly, the orientation of the awns in Niphotrichum can be useful for distinguishing between R. elongatum and R. ericoides.
Peristomes. Some species of Racomitrium sensu lato have peristome teeth that are dived to the base whereas others are only divided halfway. Some species of Schistidum, on the other hand, have peristome teeth that are strongly perforated. The orientation of papillae on the peristome teeth can also be useful for some Schistidium (e.g., S. viride).
Capsule shape. In Schistidium, the length to width ratio of capsules can be very useful for distinguishing between similar species. For example, S. rivulare has a capsule with a length:width ratio of <1.3:1 whereas S. canadense has a capsule >1.3:1.
Colony form. Do they form flattish tufts, roundish cushions, or wefts?
Plant colour (specifically for Racomitrium s. lat.). E.g., Racomitrium lanuginosum can often get quite dark in colour whereas Niphotorichum spp. are often lime green. However, this can also be useful for Schistidum and Grimmia to a degree. G. muehlenbeckii, for example, is very dark green, almost black. Most Schistidium spp. are also dark in colour but others are brown or red and some are invariably ink black (e.g., S. crassithecium).
Leaves concave vs keeled. When the plants become multistratose, the costa becomes hard to distinguish from the rest of the leaf and the leaves become more concave rather than keeled.
Margins incurved or plane vs. recurved or plane. This is a major character in the Grimmia key in FNA.
Microscopic characters:
Cross sections. (1)Laminal and Marginal Stratosity. This is a very important character, especially in sterile Grimmia and Schistidium. (2) Laminal papillosity. They are especially important for S. papillosum, S. boreale, etc., but some species intergrade and some specimens occasionally lack papillae. (3) costal shape in section.
Some notes on substrates:
I frequently find awnless forms of Schistidium spp. that are supposed to have awns on limestone in eastern Canada
I have only found Niphotrichum (Rhacomitrium) elongatum on limestone once; however, the region in which it occured is known for having relatively acidic soils.
I have only found Niphotrichum (Racomitrium) canescens on sand, whereas N. elongatum is more common on neutral to acidic gneisses. I'm not very convinced that N. ericoides exists, or at least in Ontario.
Jaffueliobryum raui can tolerate soils whereas J. wrightii is typically restricted to sedimentary (+/- calcareous) rocks. These are both in Ptychomitriaceae now, however.
Species that have been described as rheophytes (specialists in habitats with flowing water) aren't necessarily rheophytes throughout their entire distribution.
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