In the majority of areas, this is the commoner of the two species now segregated from the former S. magellanicum. Mark Hill’s article in Field Bryology, which you can download via a link on this page, deals nicely with the main differences. As it’s not always easy to estimate the angle of divergence of the leaves from the branch axes, the basal pore character in the branch leaves is often the most reliable and practical one for distinguishing S. medium from S. divinum when leaf orientation and branch appearance are borderline.
As you need to examine the pores in the hyalocysts at the extreme base of the leaf, where it joins the stem, great care must be taken to pull off leaves so that the basal part comes away intact – it’s all too easy to tear the leaves lower down. To then view the pores, place whole leaves on a dry slide so that the abaxial (convex) side is uppermost (like an upside-down dinghy). Quickly place a cover slip over the top and, press down firmly on it with forceps so that the leaves flatten (they are also likely to split longitudinally but that doesn’t matter). Still pressing down, gently introduce water and a good stain such as Methylene Blue or Crystal Violet using a needle pipette at the edge of the cover slip – this will be pulled underneath it and will stain the leaves. This technique makes the pores at the base much easier to see and also often helps to reduce air bubbles in the preparation.
The 2010 Field Guide only includes Sphagnum magellanicum agg.Read the Field Guide account