Growth experiments and chemical analyses are reported with plants of Xanthium pennsylvanicum, grown under known conditions of temperature and relative day length. Temperature was found to be a determining factor in influencing the time of flower primordia formation. This temperature effect, however, was closely associated with a response to relative day length. The interrelation of day length and temperature was shown to be as follows (1) high-temperature short-day plants gave indications of flowering 12-15 days after planting; (2) high-temperature long-day plants flowered 47 days from planting; (3) low-temperature short-day plants vegetated actively for 116 days; (4) low-temperature long-day plants remained actively vegetative for a minimum of 92 days. Chemical analyses of high-temperature short-day and low-temperature long-day plants resulted in ascending carbohydrate : N ratios in both cases as the plants approached flowering. A marked difference in the magnitude of the ratios was noted. Chemical analyses of seedlings germinated under high and low temperature conditions showed a marked accumulation of reducing and condensed sugar forms in the low temperature hypocotyl radicles. This accumulation suggests a physiological predetermination influencing the initial stages of growth. A bibliography and discussion of the literature connected with the influence of relative day length and temperature on plant growth and metabolic conditions accompanying flowering is given. Chemical methods of plant tissue analysis are also described.