CHAPTER SENSITIVENESS OF THE APEX OF THE TO CONTACT AND TO OTHER IRRITANTS
This chapter explains the nyctitropic or sleep movements of leaves. It should be remembered that we confine this term to leaves which place their blades at night either in a vertical position or not more than 30 from the vertical that is, at least 60 above or beneath the horizon. The leaves of some plants move during the day in a manner, which has improperly been called diurnal sleep; for when the sun shines brightly on them, they direct their edges towards it. The nyctitropic movement of the blade is generally effected by the curvature of the uppermost part of the petiole, which has often been modified into a pulvinus; or the whole petiole, when short, may be thus modified. From the facts and considerations now advanced conclude that nyctitropism, or the sleep of leaves / and cotyledons, is merely a modification of their ordinary circumnutating movement, regulated in its period and amplitude by the alternations of light and darkness.