CH. VIl] A SUDDEN CUKVATURE. 157 



bored split cork through which 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 iimjly 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', when the experimental plants 

 are at a convenient distance from the centre. 



In from 12 to 24 hours a good result should be 

 obtained. 



(201) Sudden curvature^. 



When a growing shoot is prevented from curving up 

 geotropically, the gravitation stimulus nevertheless pro- 

 duces some change, so that when freed from constraint 

 the shoot suddenly bends upwards. 



The constraint can best be applied by placing the 

 shoots horizontally on a shallow layer of damp sawdust, 

 and keeping them down with a sheet of plate glass. Or 

 they may be fixed to a sheet of cork by pins crossing over 

 the shoot like an X, one such fastening being placed at 

 each end and one in the middle ; the cork must then be 

 placed in damp air for some 6 hours, when the plants may 

 be unpinned. 



(202) After effect. 



A turgescent shoot is fixed by means of a cork into 



' It is convenient to keep 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 axis of rotation. 



^ Sachs' Arbeiten, i. p. 204. 



