44 BIOLOGY: GENERAL AND MEDICAL 



of mercury produces no effect, though contact with a 

 fibre of cotton-wool weighing only 0.00025 mgr. is suffi- 

 cient to stimulate the tendril to coil. 



The effect of mechanical stimulation of certain parts 

 of flowers by visiting insects is often shown in move- 

 ments of the stamens, or pistil, by which direct and 

 cross fertilization of the flowers is facilitated. Thus 

 the stamens of the barberry flowers are irritable and 

 when touched upon the inner surface curve toward the 

 pistil. 



FIG. 5. Mimosa. A. Position of a leaf at rest. B. Position of contraction 

 resulting from irritation. 



Among animals thigmotropism or reaction to mechani- 

 cal stimulation is almost universal. 



The amoeba falling upon the bottom of the pool in 

 which it lives or touching the surface of a glass slide 

 upon which it is observed through a microscope, reacts 

 by extending pseudopods and slowly moves from place 

 to place by the streaming of its body substance. As the 

 pseudopods impinge upon mechanical obstacles they are 

 withdrawn in favor of others whose progress is not 

 obstructed. If the active amceba be touched with a 

 needle, it immediately withdraws all of the pseudopods, 



