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GRAVITATIONAL SENSITIVENESS OF THE ROOT-TIP. 269 
The radicle, R, of the bean: is horizontal, and about 2-8 mm. 
of its tip is placed in a cavity in T, which may be conveniently 
described as the “tube.” When the root begins to curve geo- 
tropically, it is clear that the cotyledons must move, since the 
tip is fixed. It will take on the form shown in fig. 63 the 
IFig. 6 
A bean-seedling which has curved geotropically: the arrow gives the 
direction in$which the cotyledons will continue to move. 
amount of rotation being recorded by a pin stuck into the coty- 
ledons (not shown in fig. 6), which served as an index of angular 
movement, also as a counter-weight to balance the seed on the 
pin P, If the tip is the percipient of the stimulus, it is clear 
that the cotyledons will continue to describe a circle in the 
direction shown by the arrow. As a fact, there proved to be a 
strong tendency for the root to continue curving past the vertical 
in a way which points to the tip being the percipient organ for 
gravitation. Before giving the details of these experiments, 
I must give further description of the difficulties of the method. 
The first necessity in the conduct of the experiment is to keep 
the root from withering. Bean-roots will geotrope fairly well in 
a closed space in which the air is kept damp, but it is not easy 
to place an apparatus of the size here employed in damp air; and 
I adopted instead the plan of keeping the root and cotyledons 
damp by drops of water falling on them. This has the great 
disadvantage of making the geotropic curvature a very slow 
process, as will be seen in the details of some of the experiments. 
Another difficulty was to find the right material for the tube 
