Power to Adjust Parts to Surroundings 



239 



this margin has a greater interest than this, for it is characteristic 

 of animals also, including ourselves, where it offers the basis for 

 improvement of the body 

 through exercise, and of the 

 mind through education, 

 while it is the field, as well, 

 within which plays such free- 

 dom as is possessed by the 

 will. 



Phototropism has received 

 this generous measure of 

 attention because it is so 

 thoroughly typical of irri- 

 table responses in general. 

 Accordingly the remaining 

 forms of irritability can be 

 treated much more briefly. 



Hydrotropism. — If one pre- 

 pares a porous clay germina- 

 tor of the cylindrical form 

 represented in our picture 

 (figure 84) : fills it with water : 

 hangs it horizontally : fastens 

 small seeds along its sides: 

 and places it in a chamber 

 with a vapor-saturated at- 

 mosphere, then the stems 

 and the roots will grow stiff- 

 ly up and down as shown 

 by the first of the figures. 

 But if the surrounding air 

 be partially dry, then the roots will chng close to the porous 

 and water-soaked germinator, though the stems will act precisely 

 as before. In the first case the moisture is the same all around; 



Fig. 83. — The clinostat, an instrumfiit which 

 allows the effect of one-sided stimuli to be 

 neutralized through the continual slow rota- 

 tion of the plant. Note the resultant sym- 

 metrj' of the Nasturtium which has been 

 grown from seed on the instrument. 



