GROWTH AND MOVEMENT 477 
more sensitive it is; but this is by no means universally true. The re- 
action time varies from a few minutes to some hours, depending upon 
the temperature, the intensity of the light, and the general condition of 
the plant. 
Reversal. The reactions to light also are often reversed with age. 
This is especially seen in flower stalks, which at the time of blooming are 
likely to be positively phototropic, but later, during the ripening of the 
fruit, many become negatively phototropic, carrying the fruit under the 
leaves or even into crevices of the soil or rocks on which the species grows. 
Mechanism. The mechanism of the response is the same as in geo- 
tropism, and occurs in the same region; namely, that of most active 
growth, where one side grows more rapidly than the other, leading to a 
curvature whose tendency is to direct the axis into the line of the light 
rays. This inequality of growth is brought about by its acceleration on 
the convex side and by simultaneous retardation on the concave side. 
These changes in rate are not due to the fact that the rate of growth 
is retarded by light (see p. 435), for this (apparently applicable to posi- 
tive phototropism and once an accepted explanation) could not account 
for the acceleration on the convex side, nor for any of the changes in 
negative phototropism. The reaction is determined by the mechanism 
of the parts concerned and not by the direct influence of the stimulus. 
Perceptive region. In many phototropic reactions there is a distinct 
perceptive region, a propagation of the excitation, and an end reaction 
in a different region. Thus when seedlings of millet raised in the dark 
are exposed to lateral illumination, the sharp curvature that presently 
appears in the axis (" hypocotyl "), which is rapidly growing, can be 
shown by appropriate shading to owe its origin to the stimulus perceived 
by the leaf at the tip (" cotyledon ") and not to excitation of the axis 
itself. In a similar way the seedlings of oats show that though the whole 
of the subaerial part is sensitive to light, the tip is much the most so, 
and that excitation, spreading thence downward, dominates even con- 
trary excitation set up in the lower parts. 
What is perceived? Nothing is known as to the mode of perception 
or the structure of the perceptive organ. Indeed, it is not entirely cer- 
tain what sort of stimulus the plants perceive; whether it is the direc- 
tion of the rays, that is, the line of propagation of the waves, or whether 
it is inequality of the illumination of different sides. It has even been 
suggested, in casting about for something tangible, that plants distin- 
guish between the different pressures in the lighted and shaded portions ! 
