J 30 SENSITIVENESS OF EADICLES. Chap. III. 



it, pressing on it with very little force. How far such 

 abrupt changes in its former course are aided by the 

 circumnutation of the tip must be left doubtful. Thin 

 slips of wood were cemented on more or less steeply 

 inclined glass-plates, at right angles to the radicles 

 which were gliding down them. Straight lines had 

 been painted along the growing terminal part of some 

 of these jadicles, before they met the opposing slip 

 of wood ; and tlie lines became sensibly curved in 2 h. 

 after the apex had come into contact with the slips. 

 In one case of a radicle, which was growing rather 

 slowly, the root-cap, after encountering a rough slip 

 of wood at right angles, was at first slightly flat- 

 tened transversely: after an interval of 2 h. 30 m. 

 the flattening became oblique ; and after an addi- 

 tional 3 hours the flattening had wholly disappeared, 

 and the apex now pointed at right angles to its former 

 course. It then continued to grow in its new direc- 

 tion alongside the slip of wood, until it came to the 

 end of it, round which it bent rectangularly. Soon 

 afterwards when coming to the edge of the plate of 

 glass, it was again bent at a large angle, and de- 

 scended perpendicularly into the damp sand. 



When, as in the above cases, radicles encountered 

 an obstacle at right angles to their course, the terminal 

 growing part became curved for a length of between 

 ■ 3 and • 4 of an inch (8-10 mm.), measured from the 

 apex. This was well shown by the black lines which 

 had been previously painted on them. The first and 

 most obvious explanation of the curvature is, that it 

 results merely from the mechanical resistance to the 

 growth of the radicle in its original direction. Never- 

 theless, this explanation did not seem to us satisfactory. 

 The radicles did not present the appearance of having 

 been subjected to a sufficient pressure to account for 



