724 Hiley. — On the Value of Different Degrees of 
stimulus arises because the plant is held and prevented from falling. This 
outlook may be expressed in terms of any theory of geotropic perception 
that may be held ; thus, on the basis of the statolith theory, when a root is 
held horizontally the starch-grains fall to the lower side of each statocyte ; 
but if, when the geotropic member was placed horizontally, the whole was 
allowed to fall indefinitely, the starch-grains would not reach the lower side 
of the statocyte, because they would not fall faster than the statocyte as 
a whole. 
Now gravity ( g’ is an acceleratio 7 i of 32 feet or 981 cm. per sec. 
per sec., and this acceleration acts on all parts of a horizontal radicle. The 
geotropic stimulus is caused by the radicle being prevented from falling. 
If there were no gravity, the same effect could be produced on the radicle 
by moving it upwards with an acceleration of 981 cm. per sec. per sec. 
Movable starch-grains would then fall to the lower side of each statocyte 
and press upon it just as they do normally under the influence of gravity. 
Thus the radicles would tend to bend in a direction opposite to that of the 
acceleration. 
Now on the centrifugal wheel a plant-member is always tending to fly 
off at a tangent, but is prevented from so doing by being held at a constant 
distance from the centre of rotation. In this way it is subjected to a centri- 
petal acceleration , and it is this which causes the roots to bend centrifugally , 
i. e. in the direction opposite to that of the acceleration. Thus the so-called 
‘ centrifugal force 5 might more properly be expressed in terms of this 
centripetal acceleration. 
On the other hand, when we come to consider any sense-organ, the 
term ‘ centrifugal force ’ acquires a new meaning. Thus, in terms of the stato- 
lith theory, on the centrifuge the starch-grains move to the outer side and 
actually do exert a centrifugal force on the outer walls of the statocyte. 
In this sense the term ‘ centrifugal force * may be used in a general way 
without violence to scientific accuracy. Such an expression as f a centri- 
fugal force of 10 mg/, when occurring in this paper, may best be interpreted 
in terms of such a sense-organ. Thus, with the statolith theory, it expresses 
the force exerted on the outer wall of a statocyte by statoliths whose mass 
is m. Since, by any theory of gravi-perception, sensitiveness to the direction 
of gravity must presumably be due to the weight of some heavy body or 
substance, we may speak of centrifugal force (e. g. 10 mg.) and not only 
centrifugal acceleration (e. g. 10 g.), realizing that the force is exerted by 
the heavy body of mass m on the sensitive protoplasm. 
To prove the formula mgt — Ct f it was necessary first to construct 
a machine which would automatically provide the desired alternation of 
gravity and centrifugal force acting in opposite directions, 
