North American material referred to here as
Cerastium pumilum is very variable. At one extreme are plants resembling small annual forms of
C. fontanum, with relatively short, broad capsules, petals slightly longer than the sepals, and sepals that are usually red at the tips. At the other extreme are plants with relatively long, narrow capsules resembling impoverished diffuse-inflorescenced
C. glomeratum, with short petals and no red pigment. The latter are probably referable to
C. pumilum subsp.
glutinosum. B. Jonsell and T. Karlsson (2001+, vol. 2) treated
C. glutinosum as a distinct species in Scandinavia, but the correlation of characters that they gave to distinguish
C. glutinosum from
C. pumilum does not occur in most North American material that I have examined. Hence, the recognition of a single species, possibly with two subspecies, as in
Flora Europaea (T. G. Tutin et al. 1964-1980, vol. 1), appears to be more appropriate. The problem may arise from North American material having been introduced from several sources, whereas Scandinavian material may consist of two native genotypes that do not show the complete range of variation in the species.
Cerastium pumilum can look like a small annual form of C. fontanum but differs in its smaller capsules and the characteristic rather short, glandular hairs on the sepals, bracts, and inflorescence. It can be separated from C. semidecandrum by the much narrower scarious margins of the sepals and bracts and by the branching veins in the petals, which tend to be slightly longer and more conspicuous than in C. semidecandrum. Some forms of C. glomeratum have a very open inflorescence and may be confused with C. pumilum, but C. glomeratum has ten stamens, a narrower capsule, all the bracts herbaceous, and long, eglandular hairs (often mixed with glandular ones) on the bracts and sepals.
