Note on the difference between acrocarps and pleurocarps

1. General habit and mode of branching.

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.

Polytrichum piliferum Eurynchium praelongum
Typical acrocarp, Polytrichum
Typical pleurocarp, Eurynchium

2. Cell structure and nerves.

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.

3. Position of archegonia and capsules.

In 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, Breutelia chrysocoma and Mnium affine.

Note that Sphagnum mosses are in a class of their own.


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