58 FOUNDATIONS OF BOTANY 
reference to gravity,! is called geotropism. When, as in 
the case of the primary root, the effect of gravity is to 
make the part if unobstructed turn or move downward, 
we say that the geotropism is positive. If the tendency is 
to produce upward movement, we say that the geotropism 
is negative; if horizontal movement, that it is lateral. It 
was stated in the preceding section that the direct cause 
of the downward extension of roots is unequal growth. 
We might easily suppose that this unequal growth is not 
due to gravity, but to some other cause. To test this sup- 
position, the simplest plan (if it could be carried out) would 
be to remove the plants studied to some distant region 
where gravity does not exist. This of course cannot be 
done, but we can easily turn a 
young seedling over and over 
ia “th so that gravity will act on it 
now in one direction, now in 
4 2 another, and so leave no more 
impression than if it did not act 
Ae at all (Exp. XX). Or we can 
of / whirl a plant so fast that not 
only is gravity done away with, 
Fic. 27.—Sprouting Peas, on the Disk but another force is introduced 
of a rapidly Whirling Clinostat. jn its place. If a vertical wheel, 
Re ial hased feos like a carriage wheel, were pro- 
axis about which they were re- vided with a few loosely fitting 
Sale iron rings strung on the spokes, 
when the wheel was revolved rapidly the rings would all 
fly out to the rim of the wheel. So in Fig. 27 it will be 
1 Gravity means the pull which the earth exerts upon all objects on or 
near its surface. , 
o 
et 
