170 KNIGHT'S EXPERIMENT. [CH. vn 



disc, but it must be remembered that the roots will curve 

 away from the centre of rotation ; allowance must 

 therefore be made for their growth in that direction. 

 Roots curve perfectly well even when covered by a layer 

 of wet sponge pinned completely over them, an arrange- 

 ment which insures their being kept damp. 



The scape of Taraxacum or cabbage-shoots may be 

 used for apogeotropic curvature. Each shoot is fixed in a 

 bored cork through which, and through the contained shoot, 

 two strong pins are forced into the cork. 



It is well to devote one quarter of the space inside the 

 bell-jar to a piece of dripping wet sponge pinned firmly to 

 the cork ; this serves to keep the air moist. 



The apparatus should be made to turn at 600 or 700 

 revolutions a minute which gives a centrifugal force equal 

 to about six times gravity 1 , when the experimental plants 

 are at a convenient distance from the centre. 



In from 12 to 24 hours a good result should be 

 obtained. 



In Pfeffer's laboratory an instrument is used 2 which 

 has the advantage of giving a high centrifugal force 

 without excessive rapidity of rotation. The rotating body 

 being 150 cm. in diameter, it is possible to fix plants at 

 various distances, up to 75 cm. from the centre of rotation, 

 and thus to experiment with a graduated series of stimuli 

 at the same time. 



1 It is convenient to fcep a table from which the centrifugal force 

 can at once be calculated from the revolutions per minute and the 

 distance of the plant from the centre of rotation. 



2 See Fr. Schwarz, in Untersuchungen aus dem botan. Instititt zu 

 Tilbingen, i. 1881, p. 57. 



