Young Biloxi soy beans were grown for about a month on a long photoperiod. These long-day plants were divided into 8 series, each receiving a different short photoperiod. Plants from each series were returned to natural day after 1, 2, 3, 4, 5, 6, 8, and 10 days. Stimulation of 2 short photoperiods was sufficient to alter the course of development of the growing points in such a way that differentiation of flower primordia resulted. The length of the photoperiod influenced the time of blossoming, since the plants receiving intermediate photoperiods blossomed earlier than those receiving either the extremely long or short photoperiods used in this exp. The number of days that the plants received these various treatments influenced the time of blossoming. Plants treated for 8 days blossomed earlier than those treated for only 6 days. Meristems of plants growing on long day are described and compared with the meristems of those growing on short day. The first visible response to short photoperiod occurred in the axil of the fourth leaf primordium from the tip of the main stem and in a similar position in certain axillary buds. The region of quickest morphological response at the time the treatment is applied is an undifferentiated meristem. An after-effect of photoperiodic treatment expresses itself in the total number of compound leaves produced on the main stem. Treatments of less than 8 days did not suppress the addition of compound leaves.