128 THE VEGETATIVE FUNCTIONS OF PLANTS 
rection of the "pull" (centrifugal tendency), while the 
shoot will curve in the opposite direction (Fig. 85). 
The causal relationship between gravity and the direc- 
tion of growth of roots and shoots was first established 
by the English botanist, Thomas Andrew Knight, who 
devised the experiment illustrated in Fig. 78. 
128. Geotropism. Careful thought about these results 
will make it clear that the horizontally placed root, in 
the first experiment, does not merely bend down because 
of its weight. If this were so, we would expect the shoot 
to bend down also. The curving is the response of the 
organs to the stimulus of the pull. The property of an 
organ by virtue of which it may detect the direction of 
the pull of gravity is geotropism. It is thus seen that 
geotropism is a particular kind of irritability. Organs 
which respond by a curvature in the direction of the pull 
are positively geotropic; those which respond by a curvature 
in the opposite direction are negatively geotropic. 
129. Zone of Curvature. The following simple experi- 
ment shows that the geotropic curvature always takes 
place in a definite region. A germinating bean or other 
seed, with the sprout (hypocotyt) about 15 to 20 milli- 
meters long, is pinned to a strip of cork, fastened to the 
bottom of a Petri dish (Fig. 86). The sprout is marked 
with fine lines of India ink 2 millimeters apart, beginning 
2 millimeters back from the tip, as in the study of growth 
(page 118). Up to this point in the operations care must 
be exercised to keep the sprout as nearly parallel with the 
plumb-line as possible. By rotating the cork, or the 
entire Petri dish, the sprout is now fixed at right angles 
to the plumb-line, and the Petri dish covered and set in 
