THE MANIFESTATIONS OF LIFE 



43 



wind upon the conjunctiva is accompanied by lachry- 

 mation or rapid secretion of tears in most human beings. 



THIGMOTROPISM OR RESPONSE TO MECHANICAL 



STIMULATION. 



External agents of indifferent chemical and electrical 

 quality, and free from temperature variations to which 

 their effects can be referred, excite varying reactions in 

 living organisms according to the simplicity or complexity, 

 activity or passivity of 

 the organism stimulated. 



Animal and vegetable 

 organisms differ in their 

 manifestations, the gen- 

 eral freedom of motion 

 among the animals as 

 contrasted with the re- 

 stricted movements of 

 vegetables serving to ex- 

 plain the indifference of 

 most vegetable forms of 

 life to the effects of me- 

 chanical stimulation. 



Examples of thigmotropism among plants are, how- 

 ever, not wanting. Thus, Mimosa resents a very slight 

 mechanical irritation by closing its leaves, and Dionaea 

 closes its leaves to entrap its insect victim as soon as 

 more than one of the little hairs upon the surface have 

 been disturbed. An insect alighting upon a leaf of 

 Drosera excites the neighboring tentacles to curve upon 

 and capture it. The tendrils of climbing plants are 

 highly susceptible to the mechanical stimulation of 

 bodies with which they come into contact. "Pfeffer 

 found that they were not induced to coil by every touch, 

 but only through contact with the uneven surface of 

 solid bodies. Raindrops, consequently, never act as a 

 contact stimulus; and even the shock of a continual fall 



FIG. 4. Leaves of Sundew, a, Tentacles 

 closed over captured prey; 6, only half of 

 the tentacles closed. Somewhat magni- 

 fied. (After Darwin.) 



