HOW PLANTS USE AMIMALS. 2 3 



three minute pointed filaments standing erect and forming a tri- 

 angle on each side of the midrib. They are extremely sensitive 

 and when touched by an insect the leaf suddenly closes on it. 



The marginal spines are so placed that when the lobes close 

 they interlock like the teeth of a rat-trap- The upper surface of 

 the Itat'is covered with small reddish glands, the remainder of it 

 is c(jl()red green. The sensitive filaments are about one-twentieth 

 of an inch long and taper to a point. A peculiarly formed articu- 

 lation at the base unites them to the leaf and permits them to lie 

 flat down when the lobes close together. Their extreme sensitive- 

 ness excites the astonishment of experimenters. Darwin fixed a 

 piece of very delicate human hair into a handle, and cut it off 

 so that one inch projected; the length being sufHcient to support 

 itself in a nearly horizontal line. The extremity was then brought 

 by a slow movement laterally into contact with the tip of a fila- 

 ment, and the leaf instantly closed.* Though so sensitive to 

 touch they are utterly indifferent to heavy showers of rain and 

 gales of wind. "We thus see," says Darwin, ''that the sensitive- 

 ness of the filaments is of a specialized nature, being related to a 

 momentary touch rather than to a prolonged pressure; a touch 

 must not be from fluids, such as air or water, but from some solid 

 ol)iect." 



The surface of the blade is very slightly sensiiive, and bits 

 of stone, glass, and other inorganic substances — also bits of 

 organic substances not containing soluble nitrogenous matter 

 such as wood, cork, moss — or bodies containing soluble nitrogen- 

 ous matter, if perfectly dry, as meat, gelatine, albumen, may be 

 placed on the lobes and left for hours without producing any 

 perceptible eflect. If the nitrogenous bodies are slightly mois- 

 tened, the lobes close overtheni with a slow and gradual motion, 

 very different from that produced by touching filament. 



Tlie upper surface of the lobes is thickly covered with small, 

 sessile glands, capable of secretion and absorption. When a 

 bit of meat or an insect is enclosed, these glands pour out a 

 copious secretion, which is almost colourless, slightly mucilagi- 



*Darwin, Insectivorous Plants, p. 289. 



