Page:Life Movements in Plants.djvu/58

28 to a transient reaction, while it takes a long time for temperature variation to react on the geotropically active tissue in the interior. The persistent effect therefore takes place after a latent period from one to three hours according to the thickness of the plant.

The persistent effect of rise of temperature is a movement downwards, that of fall of temperature is a movement upwards. These definite reactions will be seen exhibited in Figs. 8 and 9. The plant was stationary at the turning point in the morning hence the curve at first was horizontal. The temperature was gradually lowered through 5°C, from 29°C, to 24°C. in the course of five minutes and maintained at the lower temperature. There was no immediate effect, but after a latent period of 65 minutes the plant responded by a movement of erection. The natural movement at this period of the day would have been one of fall, but artificial change of temperature in the opposite direction effectively reversed the normal diurnal movement. The latent period for this reverse