Acrocarpous mosses are usually unbranched or almost so, and have an erect habit, like small trees. They are never regularly pinnately (fern-like) branched but have a central stem and leaves coming off that stem. Almost all pleurocarpous mosses are freely branched, often either pinnate or chaotic. They frequently form dense intricate mats of elaborately branched sterns.
Typical acrocarp, Polytrichum
Typical pleurocarp, Eurynchium
If in doubt, look at a leaf with a compound microscope. Very few acrocarpous mosses have long narrow cells throughout the leaf, unlike pleurocarps. Few pleurocarpous mosses have short (isodiametric - as long as broad) cells in the upper part of the leaf and elongated rectangular cells in the leaf base; but in many acrocarpous families this is the usual type of cell structure. A further useful point: nerveless leaves are very rare among acrocarps; leaves with excurrent (sticking-out) nerves are almost equally rare in the pleurocarpous mosses.
almost all the acrocarpous series the archegonia - and hence the
capsules - arise terminally (at the tip of a stem or branch),
whilst in the pleurocarpous mosses they come out of side of a
stem or branch. Acrocarpous mosses that are particularly likely
to be mistaken for pleurocarps are Cinclidotus fontinaloides,
chrysocoma and Mnium affine.
Note that Sphagnum mosses are in a class of their own.
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