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 forward 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 



