174 SENSITIVENESS OF THE APEX CHAP. HI. 



slightly towards the cauterised side on the first day, and con- 

 tinuing to do so for the next three days, when the deflection 

 amounted to about 90 from the perpendicular. The cause 

 appeared to lie in the tendril-like sensitiveness of the upper part 

 of the radicle, against which the point of a large triangular flap 

 of the seed-coats pressed with considerable force; and this 

 irritation apparently conquered that from the cauterised apex. 



These several cases show beyond doubt that the 

 irritation of one side of the apex, excites the upper 

 part of the radicle to bend slowly towards the opposite 

 side. This fact was well exhibited in one lot of five 

 seeds pinned to the cork-lid of a jar ; for when after 

 6 days the lid was turned upside down and viewed 

 from directly above, the little black marks made by the 

 caustic were now all distinctly visible on the upper 

 sides of the tips of the laterally bowed radicles. 



A thin slice was shaved off with a razor from one 

 side of the tips of 22 radicles, in the manner described 

 under the common bean ; but this kind of irritation 

 did not prove very effective. Only 7 out of the 22 

 radicles became moderately deflected in from 3 to 5 

 days from the sliced surface, and several of the others 

 grew irregularly. The evidence, therefore, is for from 

 conclusive. 



Quercus robur: Sensitiveness of the apex oftlie Radicle. 

 The tips of the radicles of the common oak are fully 

 as sensitive to slight contact as are those of any plant 

 examined by us. They remained healthy in damp air 

 for 10 days, but grew slowly. Squares of the card- 

 like paper were fixed with shellac to the tips of 15 

 radicles, and ten of these became conspicuously bowed 

 from the perpendicular and from the squares ; two 

 slightly, and three not at all. But two of the latter 

 were not real exceptions, as they were at first very 

 short, and hardly grew afterwards. Some of the more 



