44 Nature of the Formative Stimulus 



respond fully with the reflex movements of animals ; that 

 is, in which one can distinguish as in animals a region of 

 perception and another of motility, as well as the 

 transmission of a stimulus from the perceptive region to 

 the motor region. 



Leaving aside the very well known example of 

 Mimosa it suffices to recall for example, that in certain 

 plants one can demonstrate that the sensibility of the 

 root to gravitation resides in its extreme tip while 

 the bending movements of that same root in order to 

 resume its vertical position after it has been disturbed 

 takes place in another part. In the same way the vertical 

 position of the stem is maintained. But a yet more 

 typical example is furnished by the grass, Setaria. "It 

 has a remarkable manner of germination; as soon as the 

 seed germinates it does not produce a simple cylindrical 

 stem but one terminating in a wedge shaped tip like a 

 lance head. When a group of Setaria is lighted from 

 one side it inclines strongly toward that side and all the 

 lance tips point toward the light. But these tips are not 

 curved at all, on the contrary the whole bending is 

 produced in the stem, although it is clearly these tips 

 which are sensitive to light and not the stems. It is easy 

 to prove this by covering the tops of some stems with 

 an opaque cap: the grass stalks so protected remain 

 vertical while others incline their stems toward the light. 

 The part that bends has not then any sensitiveness to 

 light and the part sensitive to light does not bend. The 

 little lance is the organ of perception, the stem the 

 motile region, and it is clear that a stimulus is transmitted 

 from the tip to the stem." 22 



22 Francis Darwin: Le mouvement chez les plantes. Revue 

 scientifique. March I, 1902. P. 265. 



