Effects of Sudden Changes of Temperature on Elongation Rate of Primary Root

1. The period of time roots were exposed to low temperatures influenced their elongation rate when returned to 20⚬ C. After two or three 24 hour periods the elongation rate tended to return to the elongation rate of the controls. 2. The hour of the day at which root tips were exposed to low temperatures determined the elongation rate of the roots when transferred to 25⚬C. The roots chilled when more cells were in prophase were retarded in elongation rate. Cells formed by mitotic activity of the embryonic cells started to elongate within the first 24 hours after their formation. Other factors may influence the results obtained. 3. Exposing roots grown at 15⚬ C. to high temperatures produced results similar to those produced in roots exposed to low temperatures. 4. The age of roots and their initial length did not affect the elongation rate of the roots which had been exposed to low temperatures.