On the Value of Different Degrees of Centrifugal 
Force as Geotropic Stimuli. 
BY 
W. E. HILEY, M.A. 
( Demonstrator in Botany in the University of Oxford). 
With Plates LVII and LVIII and three Figures in the Text. 
Historical. 
HE first plant physiologist to use a centrifugal wheel was Knight 
X ( 1806 ). He connected wheels rotating in horizontal and vertical 
planes with an improvised water-wheel in his garden ; and the water 
served both to drive the wheel and to keep the seedlings moist with 
incessant spray. Knight worked with seedlings of the ‘ garden bean and 
he found that when they were growing on a wheel with a horizontal axis 
the radicles all turned outwards and the ‘ germens ’ inwards, i. e. towards 
the axis. But when placed on a wheel which turned about a vertical axis 
the position taken up by the plant-members was dependent on the rate of 
rotation of the wheel. When the centrifugal force was about equal to 
gravity the radicles bent downwards and outwards at an angle of 45 0 ; and 
as the force was increased the radicles grew nearer and nearer to the 
horizontal. Knight described his chief discovery as follows : ‘ I conceive 
myself to have fully proved that the radicles of germinating seeds are made 
to descend and their germens to ascend by some external cause ; and not by 
any power inherent in vegetable life : and I see little reason to doubt that 
gravitation is the principal, if not the only agent employed, in this case, by 
nature.’ He then proceeded to propound an ingenuously mechanistic 
theory to account for the facts. 
But Knight also discovered another very important fact, viz. that 
gravity as a geotropic stimulus could be replaced by centrifugal force. 
This discovery is of extreme value because it enables the investigator 
to substitute a stimulus which can be varied at will for one which is 
necessarily fixed. 
Wigand (’ 54 ) carried out a series of experiments with a centrifugal 
wheel, placing seedlings along a radius at different distances from the centre 
(3" to 7"), with the wheel rotating at various rates (75 to 388 revolutions 
per min.) ; he found a marked disparity between the direction assumed 
by the radicles and the calculated direction of the resultant. There was in 
one case as great an angle as 38° between the two directions, but this and 
Annals of Botany, Vol. XXVII. No. CVIII. October, 1913.] 
