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206 Structural and Physiological Botany, ‘es 0 ee 
dependent of external circumstances ; the motile condition — 
—but not the movements themselves—depends on the 
temperature, the degree of light, and the amount of water 
contained in the tissues. ‘They always result from a period- 
ical alternate lengthening and shortening, first of one, then 
of the other side of the motile organ. 
Periodical movements are manifested in many other 
plants, especially in a strong curving upwards and down- 
wards of the part, and then bring about what are com- 
menly known as the durnal and nocturnal, or waking and 
sleeping positions. These phenomena are exhibited es- 
pecially in the trifoliolate and pinnate leaves of many Legu- 
minose and Oxalidez, which at night depress their common 
petioles or the laminz of the leaflets, the latter lying or 
closing upon one another in a variety of ways. The flowers 
of some other plants, as, for instance, emerocallis, close 
regularly in the evening, and open in the morning. Others, 
again, like the marigold, become erect after rain ; in clear, 
dry weather, they expand their capitula between six and 
eight in the morning, and close them between four and six 
in the afternoon ; whilst when the sky is perfectly cloudy, 
or in rainy weather, they remain altogether closed. , 
The regularity with which these movements are in general performed 
is so great that Linnzeus invented from them a ‘ floral clock,’ on which, 
however, too much dependence must not be placed. He made a list 
of a number of plants, the flowers of which open and close at a particular 
time of theday. Thus the flowers of the goatsbeard, 7 ragopogon pratensis, 
open from 3 to 5 A.M. ; of the chicory, Czchorium Intybus, from 4 to 5 ; 
of the dandelion, from 5 to 6; of the lettuce, after 7 ; of the pimpernel, 
Anagallis arvensis, after 8; of Calendula arvensis, from 9 to 10; of 
Hemerocallis flava, from 10 to 11; of the tiger-lily, 7zgridia pavonia, 
from 11 to 12; those of Hieracium murorum close after 2 P.M. ; of 
Anagellis arvensis after 3, &c. 
These periodical movements are often confounded with 
the curvatures of periodically motile organs, caused by their Bt 
exposure alternately to a greater and less intensity of light. = 
Thus, for example, sudden removal of light causes the { 
