CHAP. III. OF THE EADICLE OF THE BEAN. 153 



with their flat surfaces parallel to the cork-lid, so that 

 Sachs' curvature would not tend to make the hori- 

 zontally extended radicles turn either upwards or 

 downwards, and little squares of card were affixed as 

 before, to the lower sides of their tips. The result 

 was that all five radicles were bent down, or towards 

 the centre of the earth, after only 8 h. 20 m. At 

 the same time and within the same jars, 3 radicles of 

 the same age, with squares affixed to one side, were 

 suspended vertically ; and after 8 h. 20 m. they were 

 considerably deflected from the cards, and therefore 

 curved upwards in opposition to geotropism. In these 

 latter cases the irritation from the squares had over- 

 powered geotropism ; whilst in the former cases, in 

 which the radicles were extended horizontally, geo- 

 tropism had overpowered the irritation. Thus within 

 the same jars, some of the radicles were curving 

 upwards and others downwards at the same time 

 these opposite movements depending on whether the 

 radicles, when the squares were first attached to them, 

 projected vertically down, or were extended horizon- 

 tally. This difference in their behaviour seems at first 

 inexplicable, but can, we believe, be simply explained 

 by the difference between the initial power of the two 

 forces under the above circumstances, combined with 

 the well-known principle of the after-effects of a sti- 

 mulus. When a young and sensitive radicle is extended 

 horizontally, with a square attached to the lower side 

 of the tip, geotropism acts on it at right angles, and, 

 as we have seen, is then evidently more efficient than 

 the irritation from the square ; and the power of geo- 

 tropism will be strengthened at each successive period 

 by its previous action that is, by its after-effects. 

 On the other hand, when a square is affixed to a 

 vertically dependent radicle, and the apex begins to 



