[page 442]
than can the nyctitropic movements of cotyledons and leaves. In the latter
case they place themselves so that their upper surfaces may radiate at
night as little as possible into open space, with the upper surfaces of the
opposite leaflets often in contact. These movements, which are sometimes
extremely complex, are regulated, though not directly caused, by the
alternations of light and darkness. In the case of diaheliotropism,
cotyledons and leaves place themselves so that their upper surfaces may be
exposed to the light, and this movement is regulated, though not directly
caused, by the direction whence the light proceeds. In both cases the
movement consists of circumnutation modified by innate or constitutional
causes, in the same manner as with climbing plants, the circumnutation of
which is increased in amplitude and rendered more circular, or again with
very young cotyledons and leaves which are thus brought down into a
horizontal position by epinasty.
We have hitherto referred only to those leaves and cotyledons which occupy
a permanently horizontal position; but many stand more or less obliquely,
and some few upright.
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