Interrelation of Relative Day Length and Temperature

Growth experiments and chemical analyses were made with plants of Xanthium pennsylvanicum, grown under known conditions of temperature and relative day length. 2. 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. 3. 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 were found to flower 47 days after planting; (3) low-temperature short-day plants vegetated actively for 116 days before staminate flower buds were produced; (4) low-temperature long-day plants remained actively vegetative for a minimum of 92 days. 4. Chemical analyses of high-temperature short-day and low-temperature long-day plants resulted in ascending carbohydrate nitrogen ratios in both cases as the plants approached flower primordia formation. A marked difference in the magnitude of the ratios was noted. 5. Chemical analyses of seedlings germinated under high and low temperature conditions showed a marked accumulation of reducing sugars and condensed sugar forms in the low temperature hypocotyl radicles. This accumulation suggests a physiological predetermination influencing the initial stages of growth.