176 INJURY. [CH. VII 



(206) Localisation of sensitive region in Sorghum 1 . 

 Seeds of Sorghum sp. or of Setaria italica (see Exp. 215) 



are sown in cocoa-fibre and may be used as soon as the 

 hypocotyl is fairly grown. Provide glass tubes of such a 

 size that the cotyledons will fit tightly into them : in the 

 case of Sorghum pieces of straw may be found more 

 convenient than glass. The tubes are to be fixed hori- 

 zontally on a cork which is fixed by a vertical pin to the 

 inside of the stopper of a large glass jar. The jar must 

 be kept in a warm dark place. After a day or two the 

 hypocotyls will be curled up into circles or spirals, because 

 the cotyledons being the sensitive region, the gravitation- 

 stimulus continues to act, and therefore the hypocotyl 

 continues to bend. By way of a control experiment other 

 seedlings of Sorghum or Setaria should be fixed hori- 

 zontally, not supported by the cotyledons but by the 

 seed. In this case the hypocotyl will bend until the 

 cotyledons are vertical, when the gravitation-stimulus will 

 cease to act, and therefore growth will continue upwards 

 in a vertical direction. 



(207) Curvature induced by contact, injury, &c*. 



If the tip of a bean root is amputated by an oblique 

 cut with a razor, so that the growing point is laterally 

 injured, the root curves away from the injured side. The 

 beans should be pinned to the lid of a jar and may be 

 grown either in damp air or with the tips in water. It is 

 important that the temperature should not exceed 16 C. 



A similar curvature may be produced by contact. 

 Minute squares (1'5 mm. x T5) of cardboard or of very 



1 Francis Darwin, Annals of Botany, Vol. XIIT. 



2 Power of Movement, Chap. in. 



