L52 SENSITIVENESS OP THE APEX Chap. Ill, 



of the Bean, compared ivith that of Geotropism. — We 

 know that wheu a little square of card or other 

 object is fixed to one side of the tip of a vertically 

 dependent radicle, the growing part bends from it 

 often into a semicircle, in opposition to geotropism, 

 which force is conquered by the effect of the irri- 

 tation from the attached object. Radicles were there- 

 fore extended horizontally in damp air, kept at 

 the proper low temperature for full sensitiveness, 

 and squares of card were affixed with shellac on the 

 lotver sides of their tips, so that if the squares 

 acted, the terminal growing part would curve upwards. 

 Firstly, eight beans were so placed that their short, 

 young, horizontally extended radicles would be simul- 

 taneously acted on both by geotropism and by Sachs' 

 curvature, if the latter came into play ; and they all 

 eight became bowed downwards to the centre of the 

 earth in 20 h., excepting one which was only slightly 

 acted on. Two of them were a little bowed downwards 

 in only 5 h. ! Therefore the cards, afiHxed to the lower 

 sides of their tips, seemed to produce no effect; and 

 geotropism easily conquered the effects of the irritation 

 thus caused. Secondly, 5 oldish radicles, IJ inch in 

 length, and therefore less sensitive than the above- 

 mentioned young ones, were similarly placed and 

 similarly treated. From what has been seen on many 

 other occasions, it may be safely inferred that if they 

 had been suspended vertically they would have bent 

 away from the cards ; and if they had been extended 

 horizontally, without cards attached to them, they 

 would have quickly bent vertically downwards through 

 geotropism; but the result was that two of these 

 radicles were still horizontal after 23 h. ; two were 

 curved only slightly, and the fifth as much as 40° 

 beneath the horizon. Thirdly, 5 beans were fastened 



B 



