194 DIAGEOTROPISM. [CH. VII 



the position of the roots should be traced on the outside 

 of the glass with a paint-brush filled with white paint or 

 some clearly visible bright colour, such as vermilion 1 . 

 One end of the trough must now be raised on a wooden 

 block, so that the edge of the trough makes an angle of 

 about 45 or 50 with the horizon, and the curvature of 

 the secondary roots, by which they return to their original 

 position with regard to the horizontal, must be noted. 



(222) Growth of secondary roots in light. 



If the trough is left exposed to light instead of being 

 kept in the dark room the secondary roots grow more 

 obliquely downwards 2 . The same thing may, according to 

 Stahl, be observed in the runners of Adoxa moschatellina. 

 This is not due to a directive influence of light, it is 

 rather that light influences the mode of reaction to the 

 gravitation-stimulus. A somewhat similar state of things 

 is described in exp. 229. 



(223) Diageotropic flowers. 



The horizontal position assumed by the corolla-tube of 

 various species of Narcissus is due to diageotropism 3 . 

 The movement may be easily observed in Narcissus 

 poeticus by using either flowers attached to the plant or 

 cut specimen placed in water as shown in fig. 39. Select 

 a specimen in which the scape is vertical and the corolla- 

 tube approximately horizontal : place it (not necessarily 



1 Sachs, Collected Papers, n. p. 885 (from the Arbeiten, i). 



2 Stahl, Berichte d. deut. bot. Gesellsch. 1884. 



8 Vochtiug, Bewegungen der Bliithen und Fruchte, 1882. 



