The spores of Diphasiastrum sitchense germinate in the dark on a nutrient medium containing inorganic nutrients and glucose. Dark-grown prothalli develop into white, carrot-shaped gametophytes with a tapering base, constricted neck, and gametangial cap. The antheridia are large and sunken, and the archegonia have long necks with numerous neck canal cells. The tapering base has a zone of radially elongated cells that is comparable to the inner mycorrhizal zone of Diphasiastrum gametophytes from nature. Although possessing few derived sporophytic characters, D. sitchense has a typical carrot-shaped, Diphasiastrum gametophyte.