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 if a 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 A A represents the axis of rotation, T 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 h, just as in Knight's experiment. 

 But if the tip T is alone sensitive to gi'avitation the result will be 

 exactly revei-sed, the stimulus originating in T and conveyed to B 

 will produce the curvature in the direction t. We may think of 

 the line ilA 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'8 Jahrb. XL. 1904, p. 94. 



