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 
a 
*’Francis Darwin: Le mouvement chez les plantes. Revue 
scientifique. March 1, 1902. P. 26s. 
