Seedlings of 10 tree species were tested under two widely different photo periodic conditions in environment otherwise near optimum. The amount of light given to the plants was the same under both conditions. Under the conditions simulating long photoperiods, the seedlings of all the species grew well and continued their growth during the 8-month experimental period. Under the short photoperiod, the following species remained nearly or fully dormant for several months beginning soon after germination:Larix laricina(Du Roi) K. Koch,Ulmus americanaL., and two provenances ofPicea glauca(Moench) Voss; the following species either remained nearly dormant or grew significantly slower than under the long day conditions:Betula luteaMichx. f.,B.verrucosaEhr., andPinus banksianaLamb.;P.resinosaAit. was considered sensitive to photoperiods but its responses in appearance and growth were small;Caragana arborescensLam.,Thuja plicataDonn, andAcer negundoL. showed no significant response.