GROWTH AND MOVEMENT 455 
the motor organ lies along the central rib, between the two lobes of the 
leaf, and when an insect touches one of the three sensitive bristles on 
either face, these lobes shut together quickly like the jaws of a trap, 
and their interlocking teeth prevent the prey from crawling out easily. 
After a time the superficial glands pour out a secretion containing an 
enzyme that digests the proteins, and these are absorbed and utilized 
as food. After several days the trap again opens. Somewhat slower 
movements are made by the " tentacles " of Drosera (fig. 685). When 
an insect is entangled in the viscid secretion at the tips of these leaf lobes, 
its struggles furnish a stimulus which results in the incurving of all, until 
it is completely enveloped in their secretion, which then changes char- 
acter, becoming digestive, and so prepares the proteins for absorption 
(see p. 388). 
Gravity movements. Gravity cannot act as a stimulus unless the 
plant be displaced. If a potted bean plant be turned upside down or 
laid on the side, in a few hours the motor organs become curved so 
as to bring the leaves again into the usual position, or as near to it as 
possible. 
Photeolic movements. The most striking movements are the regular 
ones produced by motor organs under periodic stimulation by variations 
in the intensity of light (and temperature). These have been known 
under the misleading name of " sleep movements," because they are 
notable at nightfall. However, they have no similarity whatever to the 
relaxed position assumed by animals in sleep, nor do they bring any 
recovery from fatigue. On the contrary, the nocturnal position is one of 
precisely as much strain as the diurnal one, since the resistance of the 
motor organ to bending is measurably the same; and even the position 
is as likely to be erect as drooping. 
Technically they have been called nyctitropic movements, but as the curvature 
is not a tropic one this term is objectionable, and the more so as the movements are 
quite as much associated with day as with night. They are best called photeolic 
(i.e. light variation) movements, because the illumination is chiefly responsible for 
them, though corresponding fluctuations in temperature accompany the changes 
in light and sometimes cooperate in setting up the movement. 
Photeolic movements consist of a rising or falling, a fonvard or back- 
ward movement, of the entire leaf and (if the leaf be compound) of all 
the leaflets as well; or the leaflets alone of a compound leaf may exhibit 
such movements. The change in the leaves of the common purslane 
(figs. 686, 687) will make clear the general character of these changes 
