This exceedingly widespread and variable species is common in the flora region. Two subspecies are recognized in North America (Mosquin, 1966) based on a combination of quantitative leaf and pollen characters, geographical range, and chromosome number. The diploid (with n = 18) subsp. angustifolium, with a more northern subarctic distribution, has smaller, glabrous leaves, and smaller triporate pollen. The polyploid (with n = 36, 54) subsp. circumvagum Mosquin is distributed farther south in the boreal forest zone, and has larger, sometimes pubescent leaves, and larger pollen, some of which is quadriporate. The pattern of variation in the Himalayan region is not so clearly defined, either geographically or morphologically, and there is no significant number of chromosome counts. It seems best, therefore, to treat this species in the Himalayan region without subspecific rank, at least until more critical studies have been carried out. Fl. Per.: Jul-Sep. Fr. Per.: Aug-Sep.
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