Bryum ruderale is a very common plant, but much ignored and possibly misrecorded for similar small tuber-forming species in Bryum Section Bryum. Microscopic examination of leaves, rhizoids and rhizoidal tubers is normally essential. See our Tips and Techniques page to learn how to remove soil from the rhizoids in order to see these structures. Note that when examining rhizoids for colour and papillosity, it is important to select only large rhizoids, as smaller ones are often a different colour.
Candidate B. ruderale may be selected in the field on account of its large (>100um diameter) spherical red to purplish-red tubers with a smooth outline and deep violet rhizoids. Under a microscope, the larger rhizoids will be seen to be strongly papillose. B. violaceum also has violet rhizoids but they will appear smooth or only slightly papillose. Its spherical red tubers are usually around half the diameter of those of B. ruderale.
Bryum rubens has red rhizoidal tubers of a similar size, but they have protruberant cells and are borne on rhizoids of various colours, but never deep violet. Its tubers are also often clustered above the soil surface, a feature not seen in B. ruderale.