Fig. 119.
Sunflower plant. Epinastic condition of
leaves induced during the day in darkness.
Fig. 120.
Sunflower plant removed from darkness,
leaves extending under influence of light
(diaheliotropism.)
265. Epinasty and hyponasty.—During the early stages of growth of many leaves, as in the sunflower plant, the direction of growth is different from what it is at a later period. The under surface of the young leaves grows more rapidly in a longitudinal direction than the upper side, so that the leaves are held upward close against the bud at the end of the stem. This is termed hyponasty, or the leaves are said to be hyponastic. Later the growth is more rapid on the upper side and the leaves turn downward or away from the bud. This is termed epinasty, or the leaves are said to be epinastic. This is shown by the night position of the leaves, or in the induced “sleep” of the sunflower plant in the experiment detailed above. The day position of the leaves on the other hand, which is more or less horizontal, is induced because of their irritability under the influence of light, the inherent downward or epinastic growth is overcome for the time. Then at nightfall or in darkness, the stimulus of light being removed, the leaves assume the position induced by the direction of growth.
Fig. 121.
Squash seedling. Position
of cotyledons in light.
Fig. 122.
Squash seedling. Position
of cotyledons in the dark.
266. In the case of the cotyledons of some plants it would seem that the growth was hyponastic even after they have opened. The day position of the cotyledons of the pumpkin is more or less horizontal, as shown in [fig. 121]. At night, or if we darken the plant by covering with a tight box, the leaves assume the position shown in [fig. 122].