THE WONDERS OF LIFE 



very delicate feeling for the nature of the supports which 

 they embrace; they distinguish between smooth and 

 rough, thick and thin supports, and prefer the latter. 

 Many of the higher plants, which are particularly sensi- 

 tive to pressure, have, to an extent, special organs of 

 touch (tentacles), and reveal this by the movements of 

 their leaves (the sensitive plants, mimosa, dionosa, oxalis). 

 But even among the unicellular protists we find that the 

 contact of solid bodies has an irritating effect, the per- 

 ception of which provokes corresponding movements 

 {thigmotaxis or thigmotropismus) . A peculiar form of 

 pressure-sensation is produced in many organisms by 

 the flow of liquids; in the mycetozoa, for instance, it 

 provokes counter-movements {rheotaxis, rheotropismus), 

 as Ernst Strahl showed by his experiments on cBthelium 

 septicum. 



We have an interesting analogy to the thigmotaxis of 

 the viscous living plasm in the elasticity of solid inor- 

 ganic bodies, such as an elastic steel-rod. In virtue of 

 its springy nature, the elastic rod reacts on the pressure 

 of force that has bent it, and endeavors to regain its 

 former position. The spiral spring sets the works of the 

 clock in motion in virtue of its elasticity. 



A very important part is played in botany by the 

 action of gravitation on the growth of plants. The 

 attraction towards the centre of the earth causes the 

 positively geotropic roots to grow vertically into the 

 earth, while the negatively geotropic stalk pushes out 

 in the opposite direction. This applies also to a number 

 of stationary animals which are attached to the ground 

 by roots, such as polyps, corals, bryozoa, etc. And even 

 the locomotion of free animals, the disposition of their 

 bodies to the ground, the position and posture of their 

 limbs, etc., is determined partly by the feeling of 

 gravitation, and partly by adaptation to certain functions 

 which resist this, as in running, swimming, and so on. 



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