Root-tip 395 
less striking result. Although these researches confirmed Darwin's 
work on roots, much stress cannot be laid on them as there are 
several objections to them, and they are not easily repeated. 
The method which—as far as we can judge at present—seems 
likely to solve the problem of the root-tip is most ingenious and is 
due to Piccard’. 
Andrew Knight’s celebrated experiment showed that roots react 
to centrifugal force precisely as they do to gravity. So that ifa bean 
root is fixed to a wheel revolving rapidly on a horizontal axis, it tends 
to curve away from the centre in the line of a radius of the wheel. 
In ordinary demonstrations of Knight’s experiment the seed is 
generally fixed so that the root is at right angles to a radius, and as 
far as convenient from the centre of rotation. Piccard’s experiment 
is arranged differently. The root is oblique to the axis of rotation, 
and the extreme tip projects beyond that axis as shown in the sketch. 
The dotted line AA represents the axis of rotation, 7’ is the tip of 
the root, B is the region in which curvature takes place. If the 
motile region B is directly sensitive to gravitation (and is the only 
part which is sensitive) the root will curve away from the axis of 
rotation, as shown by the arrow 0, just as in Knight’s experiment. 
But if the tip 7 is alone sensitive to gravitation the result will be 
exactly reversed, the stimulus originating in 7 and conveyed to B 
will produce the curvature in the direction & We may think of 
the line AA as a plane dividing two worlds. In the lower one 
gravity is of the earthly type and is shown by bodies falling and 
roots curving downwards: in the upper world bodies fall upwards 
1 Pringsheim’s Jahrb. xu. 1904, p. 94. 
