-Critical length of dark periods for induction of Xanthium pennsylvanicum was found to be markedly increased by low dark-period temps., and to a lesser extent by low light intensity during photoperiods preceding long dark periods. High humidities had only a slight effect. Shorter dark periods sufficed to induce the plants as they became older. Induced leaves grafted to vegetative Xanthium plants maintained on long photoperiod brought about induction in 3 of 6 cases. A residual effect of dark period treatment insufficient to bring about induction was demonstrated to last at least 24 hrs. in Xanthium, but not in Biloxi soy beans. Two separate photoperiodic treatments of Biloxi soy beans, each sufficient to induce results, were independent of each other in effect as judged by number and position on the main axis of flower buds.