161) by Dassen in 1837.
** 'Flora,' 1873, p. 433.
*** 'Bot. Zeitung,' 1879, Dec. 19th, p. 830.
[page 284]
ance of such movements to the plant. There is another difference between
the two sets of cases, namely, that there is never, or very rarely, any
torsion of the leaves, excepting when a pulvinus is present;* but this
statement applies only to periodic and nyctitropic movements as may be
inferred from other cases given by Frank.**
The fact that the leaves of many plants place themselves at night in widely
different positions from what they hold during the day, but with the one
point in common, that their upper surfaces avoid facing the zenith, often
with the additional fact that they come into close contact with opposite
leaves or leaflets, clearly indicates, as it seems to us, that the object
gained is the protection of the upper surfaces from being chilled at night
by radiation. There is nothing improbable in the upper surface needing
protection more than the lower, as the two differ in function and
structure. All gardeners know that plants suffer from radiation.
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