72 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



turgescence of certain cells, and has nothing in common with animal 

 motion. 



The Dionxa muscipula, or Venus' Fly-Trap, furnishes another illus- 

 tration of movement of parts occurring in the vegetable kingdom. The 

 form of the bilobed leaf, which is the movable part of this plant, is 

 shown in Fig. 45. The two lobes stand at rather less than a right 

 angle to each other, and on each of the inner surfaces are three minute 

 filaments projecting inward. The margins of the leaf are prolonged 

 into spikes, into each of which a bundle of spiral vessels enters. When 

 any one of these filaments is touched, even by so slight a pressure as 

 would be produced by contact with a hair, the leaves instantly come into 

 apposition, and the spikes interlock like the teeth of a rat-trap. The 

 upper surface of the leaf is covered with minute glands, which furnish a 

 secretion having the power of digesting organic substances. When 



insects come in contact with 

 these filaments, the leaves close 

 so as to imprison them, and 

 the insects are digested by the 

 acid secretion stimulated by 

 their contact, and absorbed. 



In this plant the chief seat 

 of the movement is in the thick 

 mass of cells which overlies the 

 central bundles of vessels in 

 the mid-rib. When any one 



FIG. 45. VENUS' FLY-TRAP (Dionoea musdpuia). of these filaments on the in- 

 LEAF VIEWED LATERALLY IN ITS EXPANDED , ^^1,1 



STATE. (Darwin.) ternal surface of the leaves is 



touched the impulse travels in 



all directions through the cellular tissue, independently of the course of 

 the vessels, to the cells at the mid-rib. Fluid thus flowing from the 

 upper cells to the lower, the lower cells greatly increase in tension, while 

 the upper ones become relaxed and the leaves come into apposition. 

 Opening of the leaves is accomplished by a reverse process. In this 

 plant there is therefore to be seen not only a mechanical irritation, 

 which produces mechanical motion by purely mechanical means, but also 

 a chemical irritation through contact of various substances with the 

 leaf, which results in the production of a digestive secretion. 



2. PROTOPLASMIC MOVEMENTS. Protoplasmic movements, which may 

 be seen in both the animal and vegetable kingdoms, may be of various 

 kinds. We may meet with movements of free protoplasm, or of proto- 

 plasm while contained within cell-walls. 



The peculiarity of protoplasmic motion lies in the fact that the 

 particles of the contractile mass do not move around any fixed point, 



