494 LECTURE XIX. 



short bits of moderately thick bristle, fixed on with gum-water, 

 acted in three only out of eleven trials, and beads of shellac 

 weighing less than -^ grain acted only twice in nine cases. 

 The most interesting evidence of the delicate sensitiveness of 

 the tip of the radicle was afforded by its power of discrimin- 

 ating between equal-sized squares of card-like and of very 

 thin paper, when these were attached on opposite sides the 

 radicles curved away from the heavier object, as was observed 

 in the Bean and the Oak. 



Darwin's observations on this peculiar manifestation of 

 irritability by radicles have given rise to considerable .dis- 

 cussion. Wiesner confirms Darwin's statement that the 

 Darwinian curvature is not induced by friction, and he 

 regards it as a pathological phenomenon. With regard 

 to the experiments with attached objects, he points out 

 that the curvature cannot be due to a stimulating action 

 of the small piece of card or paper, for, as we have seen 

 already, a radicle will grow straight against a considerable 

 resistance, for instance, when it grows downward into mer- 

 cury, and he himself observed, in special experiments, that 

 radicles in their downward growth exerted a pressure of 

 about a gramme, and that radicles growing horizontally 

 pushed weights of 075 1*25 grme. out of their way without 

 becoming curved. Darwin himself has estimated the pressure 

 of a Bean-root growing straight downward at a quarter of 

 a pound. Wiesner concludes that the curvatures observed 

 in Darwin's experiments with attached objects were due, 

 not to the pieces of card, bristle, etc., but to the irritating 

 effect of the shellac. He found that when small pieces of 

 wood or grains of sand were made to adhere to radicles, 

 without the use of any adhesive material, no curvature took 

 place, and also, that the curvatures were induced equally well 

 when a drop of shellac was employed without anything else. 

 Microscopical examination shewed that the part of the tip 

 touched by the shellac had died away. Wiesner concludes 

 that the effect of irritating one side of the tip of a radicle in 

 any of the ways mentioned is to cause, in the first instance, 

 a slight concavity of the irritated side, in consequence of the 



