502 MANUAL OF BOTANY 



sj'stem in plants, we should expect to find a more or less evident 

 differentiation of it in the direction of sense organs. This 

 is in fact afforded in many cases. Darwin found that the 

 seedlings of Phalaris were not sensitive to the faint light em- 

 ployed in his experiments except at a small region extending 

 about yV iiich from the apex. If this part were covered 

 by an opaque screen in the shape of a small blackened cap, of 

 not sufficient weight to cause any flexion of the stem, the seed- 

 lings no longer bent towards the light. Similar careful experi- 

 ments made by the same observer pointed out that the tip of 

 the young root was its only sensitive part. Other observers 

 have proved the same thing. Cisielski amputated the tips of 

 certain rootlets and laid them horizontally on a support. They 

 then did not show any sensitiveness until they had recovered 

 from the wound and a new root-tip was formed on each. As 

 soon as the new tip was developed the rootlets showed a power 

 of reacting to the force of gravity, and the curvature resulted in 

 the usual place, More recently Pfeffer has demonstrated the 

 same localisation. The whole leaf of Dionaea may be somewhat 

 roughly handled without closing, so long as no contact is made 

 with the hairs upon a particular portion of the blade. But so 

 soon as one of these is touched the leaf closes. It is impossible 

 to avoid the conclusion that we have to do here with a localisa- 

 tion of sensitiveness or the differentiation of sense organs. 

 True the differentiation is anatomically very slight, but physio- 

 logically it is considerable. The same sense organ again is 

 sensitive in very different degrees to different stimuli, and, as we 

 should expect, to different strengths of the same stimulus. If a 

 sensitive organ be acted upon at the same time by two stimuli 

 which usually produce opposite movements, the resulting posi- 

 tion is always that which would be caused by the stronger of 

 the two. 



One of the characteristic features of the nervous mechanism 

 of an animal is the definite character of the response made to 

 a stimulus. This point is also brought out in considering the 

 behaviour of the parts under notice. If a root-tip is brought 

 into contact with an obstacle, the bending is invariably in such 

 a direction as to enable the root to pass it. ^Yhen one is 

 allowed to impinge upon a plate at right angles to its direction 

 of growth, the curvature continues till the root has turned 

 through a right angle and can grow parallel to the opposmg 

 surface. If on the other hand a part some little distance above 

 the tip is obstructed, the cui'vature is towards the obstacle, a 



