1. The irritability of Mimosa pudica rises rapidly during the first part of the morning, remains high throughout the daylight hours, and falls gradually during the night to a minimum around daybreak. 2. If plants are kept under continuous illumination for 24 hours, irritability does not fall during the night period as normally occurs but remains at the high daytime level. 3. If plants are kept in the dark for 24 hours, irritability does not rise during the daytime period as normally occurs but remains at the low nighttime level. 4. It is concluded that the increase in irritability during the day is due primarily to the effect of light. 5. Other investigators have found contrary to our observations that the irritability of Mimosa is lowest in the daytime and highest in the night. This contradiction in results is attributed to the use by other observers of an improper criterion and method for determining irritability.