11. Effect of temperature on mutation rate of a allele when plants were matured at two levels of temperature after fertilization. Plants of a Dt constitution were grown at a temperature of approximately 70 degrees F. until flowering. Immediately after pollination they were divided at random into two lots and one placed in a greenhouse maintained around 60 degrees F. and the second lot placed in an adjoining house maintained at or near 80 deg. F. The two lots of plants were left at the two temperature levels until seed was ripened. The mutation rates at the two temperatures were determined by counting the number of aleurone dots. The average mutation rate was determined by counting the number of dots on fifty seeds of each ear except for those ears marked by asterisks where less than fifty seeds were available. The data obtained are given below:

The results listed above are somewhat astonishing and to the writer entirely unexpected. A similar experiment is being conducted this year on a more extensive scale. If the same effect is found it should be possible to determine the critical period at which the temperature change has its effect. It also will permit inferences, or if you wish, guesses, as to the nature of the a-Dt reaction.