We endeavoured,
therefore, to ascertain something on this head by preventing the leaves
[page 286]
of several plants from going to sleep, and by exposing to a clear sky when
the temperature was beneath the freezing-point, these, as well as the other
leaves on the same plants which had already assumed their nocturnal
vertical position. Our experiments show that leaves thus compelled to
remain horizontal at night, suffered much more injury from frost than those
which were allowed to assume their normal vertical position. It may,
however, be said that conclusions drawn from such observations are not
applicable to sleeping plants, the inhabitants of countries where frosts do
not occur. But in every country, and at all seasons, leaves must be exposed
to nocturnal chills through radiation, which might be in some degree
injurious to them, and which they would escape by assuming a vertical
position.
In our experiments, leaves were prevented from assuming their nyctitropic
position, generally by being fastened with the finest entomological pins
(which did not sensibly injure them) to thin sheets of cork supported on
sticks.
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