DIAPHOTOTROPISM 137 



208. Aphototropism. Germinate seeds of Sinapis alba in saw- 

 dust, or loose soil, and when the roots are about 2 cm. in length 

 take up the seedlings and pass them through holes of proper 

 size in a thin' plate of cork, where they are supported by pack- 

 ing of cotton wool. Fill a tumbler with water to within about 

 3 mm. of the top and set the cork over the mouth with the roots 

 fully immersed in the water, and the entire axis of the plant in a 

 vertical position. Set the preparation in a room at 18 C. near 

 a window where it will receive light from one side only. This 

 may be best accomplished by placing the preparation in the photo- 

 tropic chamber from which the door has been removed and the 

 open end directed toward the light (Fig. 68). 



FIG. 69. Shoot of Helianthus which has been placed in a horizontal position and 

 illuminated from above. The diageotropic and diaphototropic movements of the 

 leaves have been accomplished by curvatures and torsions of the petioles. 



209. Diaphototropism. Probably all dorsiventral leaves tend to 

 place their axes at right angles to the incident rays of light, with 

 the inner (upper) surfaces exposed to the direct action of the rays. 

 If the leaf has an exposure including the whole horizon it will 

 lie in a horizontal position, which might be due also to diageo- 

 tropism. The crowding of leaves under the shadow of other or- 

 gans of the same plant, or of surrounding vegetation however, 

 alters its horizon, in consequence of which it assumes various 

 positions with respect to the vertical, but at right angles to the 

 direction from which its optimum illumination is derived in what 



