186 THE EFFECT OF KILLING OK Chap. lU. 



gravity likewise resides in the tip ; so that it is the 

 tip which excites the adjoining parts of a horizontally 

 extended radicle to bend towards the centre of the 

 earth. 



Skcondaey Kadioles becoming vertically Geo- 



TEOPIC BY the DEbTEUCTION OB INJURY OF THE 



Terminal Part of the Primary Eadicle. 



Sachs has shown that the lateral or secondary 

 radicles of the bean, and probably of other plants, are 

 acted on by geotropism in so peculiar a manner, that 

 they grow out horizontally or a little inclined down- 

 wards ; and he has further shown* the interesting fact, 

 that if the end of the primary radicle be cut off, one 

 of the nearest secondary radicles changes its nature 

 and grows pei"pendicularly downwards, thus replacing 

 the primary radicle. We repeated this experiment, 

 and planted beans with amputated radicles in friable 

 peat, and saw the result described by Sachs; but 

 generally two or three of the secondary radicles grew 

 perpendicularly downwards. We also modified the 

 experiment, by pinching young radicles a little way 

 above their tips, between the arms of a U-shaped 

 piece of thick leaden wire. The part pinched was 

 thus flattened, and was afterwards prevented from 

 growing thicker. Eive radicles had their ends cut 

 off, and served as controls or standards. Eight were 

 pinched ; of these 2 were pinched too severely and 

 their ends died and dropped off; 2 were not pinched 

 enough and were not sensibly affected ; the remaining 

 4 were pinched sufficiently to check the growth of 

 the terminal part, but did not appear otherwise injured. 

 When the U-shaped wires were removed, after an 



• ' Arboiten Bot. Institut., Wiirzburg,' Heft iv. 1874, p. 622. 



