IRRITABILITY. 



59 



EXPERIMENT 69. 



PROHELIOTROPISM. 



Place a Malva or Helianthus grown in a pot in the open air, 

 near a window with a southern exposure. The leaves gradually 

 assume the definite positions shown in Fig. 52. 



EXPERIMENT 70. 



HELIOTROPIC MOVEMENTS OF ROOTS AND SHOOTS. 



Fasten seedlings of Sinapis alba (Mustard) or Phaseolus multiflo- 

 rus (Bean) on a piece of tulle stretched lightly across a glass vessel 



FIG 53- 



Dark chamber with a tube opening in one end. (Schleichert.) 

 filled with spring-water. After the roots and stems have attained a 

 p IG> 5 4> length of i cm. place the apparatus 



under a pasteboard box lined with 

 black paper, through which the 

 light may gain entrance by a small 

 aperture. In a few hours the roots 

 and stems will be influenced as de- 

 scribed above. (Figs. 53 and 54.) 

 EXPERIMENT 71. 



HELIOTROPIC MOVEMENTS OF LEAVES. 



Bend and fasten in a horizontal 

 position an upright well-leaved 

 branch of the Maple, or a whole 



Helianthus plant, in the open air. 

 Seedling of Mustard grown under ... _ 



one-sided illumination. (Det- Soon the leaves which were previ- 



mer.) ously horizontal, and perpendicu- 



lar to the shoot on all sides, show peculiar torsions of the petioles, 



