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- FlG - 83- The clinostat, an instrument which 



allows the effect of one-sided stimuli to be 

 ly Up and down as Shown 



Dy the first OI the figures. 



But if the surrounding air 

 be partially dry, then the roots will cling 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; 



neutralized through the continual slow rota- 

 tion of the plant. Note the resultant sym- 

 metry of the Nasturtium which has been 



