This very small moss takes a bit of practice to recognize, as initially, it can be overlooked as the common and widespread Amblystegium serpens. It is at its most impressive on calcareous soil in woodlands where the delicate wefts of its stems can form rather lovely fine turfs. A. serpens doesn’t usually grow in quite that manner, looking a bit more scrappy, but a hand lens is needed to confirm the ID. Make sure the plant is in the moist condition as the leaves distort on drying. The leaf tip of A. serpens is characteristically drawn out into a long, almost parallel-sided point. In contrast, the sides of the leaf tip of M. pumilum converge uniformly at an angle of about 30 – 40 degrees, giving the leaf a blunter look. It really is a tiny moss though, so get some practice. Collect some candidates and confirm their identity under the microscope, by virtue of the now clear leaf-shape difference, the longer, thinner cells near the leaf tip of A. serpens and the presence of an occasional sharp point on the back of the nerve in the M. pumilum. Then look through the hand lens until you can see the difference between the two species in the hand. As well as on soil, you might find it growing over stones and tree roots, but usually in the shade. It can also be a bit scrappy like A. serpens, but once you’ve had the practice, a quick peer through the hand lens will separate the two.
As a final point, in western areas you may encounter Heterocladium heteropterum which can look a little like M. pumilum. The lens will reveal, however, that the Heterocladium species has hardly any trace of a nerve and duller, less translucent leaves.Read the Field Guide account