There is only one
other analogous case known to us, namely, that of certain Gramineae, which
fold inwards the sides of their narrow leaves, when these are exposed to
the sun and to a dry atmosphere, as described by Duval-Jouve.* We have also
observed the same phenomenon in Elymus arenareus.
* 'Annal. des Sc. Nat. (Bot.),' 1875, tom. i. pp. 326-329.
[page 414]
There is another movement, which since the time of Linnaeus has generally
been called sleep, namely, that of the petals of the many flowers which
close at night. These movements have been ably investigated by Pfeffer, who
has shown (as was first observed by Hofmeister) that they are caused or
regulated more by temperature than by the alternations of light and
darkness. Although they cannot fail to protect the organs of reproduction
from radiation at night, this does not seem to be their chief function, but
rather the protection of the organs from cold winds, and especially from
rain, during the day. the latter seems probable, as Kerner* has shown that
a widely different kind of movement, namely, the bending down of the upper
part of the peduncle, serves in many cases the same end.
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