164 SENSITIVENESS OF THE APEX CHAP. III. 



of various sizes of the same card-like paper, also with 

 bits of thin glass and rough cinders, affixed with shellac 

 to one side of the apex. Bather large drops of the 

 dissolved shellac were also placed on them and allowed 

 to set into hard beads. The specimens were subjected 

 to various temperatures between 60 and 72 F., more 

 commonly at about the latter. But out of this con- 

 siderable number of trials only 5 radicles were plainly 

 bent, and 8 others slightly or even doubtfully, from 

 the attached objects ; the remaining 46 not being at 

 all affected. It is therefore clear that the tips of the 

 radicles of this Phaseolus are much less sensitive to 

 contact than are those of the bean or pea. We 

 thought that they might be sensitive to harder 

 pressure, but after several trials we could not devise 

 any method for pressing harder on one side of the 

 apex than on the other, without at the same time 

 offering mechanical resistance to its growth. We 

 therefore tried other irritants. 



The tips of 13 radicles, dried with blotting-paper, 

 were thrice touched or just rubbed on one side 

 with dry nitrate of silver. They were rubbed thrice, 

 because we supposed from the foregoing trials, that 

 the tips were not highly sensitive. After 24 h. the 

 tips were found greatly blackened ; 6 were blackened 

 equally all round, so that no curvature to any one 

 side could be expected; 6 were much blackened on 

 one side for a length of about -Vth of an inch, and 

 this length became curved at right angles towards the 

 blackened surface, the curvature afterwards increasing 

 in several instances until little hooks were formed. 

 It was manifest that the blackened side was so much 

 injured that it could not grow, whilst the opposite 

 side continued to grow. One alone out of these 13 

 radicles became curved from the blackened side, the 



