CHAP. XIII. 
IRRITABILITY. 
347 
shelter those most delicate organs from the chill air of night 
The flowers of the crocus and similar plants expand beneath 
the bright beams of the sun, but close as soon as they are 
withdrawn. The CEnotheras unfold their blossoms to the dews 
of evening, and wither away at the approach of day. Some 
Silenes roll up their petals in the day, and expand them at 
night. The florets of numerous Compositae, and the petals of 
the genus Mesembryanthemum, are erect in the absence of 
sun, but become reflexed when acted upon by the sun's beams ; 
and many other such phenomena are familiar to every observer 
of nature. It is probable, indeed, that a different effect is 
produced upon all plants by day and night, although it is less 
visible in some than in others : thus plants of corn, in which 
there is little indication of sleep when grown singly, exhibit 
that phenomenon very distinctly when observed in masses ; 
then- leaves become flaccid, and their ears droop at night. 
These effects have been generally attributed to the action of 
light; and it is probable that that agent contributes very 
powerfully to produce them ; for a flower removed from the 
shade will often expand beneath a lamp, just as it will beneath 
the sun itself De Candolle found that he could induce plants 
to acknowledge an artificial day and night, by alternate expo- 
sure to the light of candles. There must, however, be some 
cause beyond light, of the nature of which no opinion has yet 
been formed : many flowers will close in the afternoon while 
the light of the sun is still playing upon them, and the petals 
of others will fold up under a bright illumination. 
Spontaneous movements are far more uncommon than those 
which have just been described. In Megaclinium falcatum, 
the labellum, which is connected very slightly with the 
columna, is almost continually in motion ; in a species of 
Pterostylis, shown me by Dr. Brown, I observed a kind of 
convulsive action of the labellum ; the filaments of Oscilla- 
torias are continually writhing like worms in pain ; several 
other Confervas exhibit spontaneous movements : but the 
most singular case of the kind is that of Hedysarum gyrans. 
" This plant has ternate leaves : the terminal leaflet, which is 
larger than those at the side, does not move, except to sleep ; 
but the lateral ones, especially in warm weather, are in con^ 
