CH. VIl] SUDDEN CURVATURE. 171 



(201) Sudden curvature^. 



When a growing shoot is prevented from curving 

 apogeotropically, the gravitation stimukis nevertheless 

 produces some change, so that when freed from constraint 

 the shoot suddenly bends upwards. 



The constraint may 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 

 a bottle of water so placed that the shoot projects horizon- 

 tally. A needle to serve as an index is fixed in the free 

 end of the shoot and its position recorded on a vertical 

 scale. After about an hour, — or when the shoot has 

 begun to curve apogeotropically, — the bottle is rotated 

 on its axis through 180°, so that the plane of curvature 

 remains vertical, but what was the upper side of the 

 shoot is now the lower. The index will now travel 

 downwards over the scale, owing to the continuance of 

 the curvature induced by the gi-avitation stimulus. It 

 will finally come to rest and will at last curve up in tlie 

 opposite direction ^ 



^ Sachs' Arbeiten, i. p. 204. 



2 Sachs' Collected Papers, ii. p. 966 (from Flora, 1874). 



