550 SUMMARY AND CHAP. XU. 



or oblique fissure in the earth, or a burrow made by 

 an earth-worm or larva; and it is certain that roots 

 often run down the old burrows of worms. The tip, 

 however, in endeavouring to circumnutate, will con- 

 tinually press against the earth on all sides, and this 

 can hardly fail to be of the highest importance to the 

 plant ; for we have seen that when little bits of card- 

 like paper and of very thin paper were cemented on 

 opposite sides of the tip, the whole growing part oi 

 the radicle was excited to bend away from the side 

 bearing the card or more resisting substance, towards 

 the side bearing the thin paper. We may therefore 

 feel almost sure that when the tip encounters a stone 

 or other obstacle in the ground, or even earth more 

 compact on one side than the other, the root will bend 

 away as much as it can from the obstacle or the more 

 resisting earth, and will thus follow with unerring 

 skill a line of least resistance. 



The tip is more sensitive to prolonged contact with 

 an object than to gravitation when this acts obliquely 

 on the radicle, and sometimes even when it acts in the 

 most favourable direction at right angles to the radicle. 

 The tip was excited by an attached bead of shellac, 

 weighing less than ^o^ n f a g rain (0'33 mg.) ; it is 

 therefore more sensitive than the most delicate ten- 

 dril, namely, that of Passiftora gracilis, which was barely 

 acted on by a bit of wire weighing ^th o f a g ra i n . But 

 this degree of sensitiveness is as nothing compared with 

 that of the glands of Drosera, for these are excited by 

 particles weighing only y-gy^o f a g rain - The sensi- 

 tiveness of the tip cannot be accounted for by its 

 being covered by a thinner layer of tissue than the 

 other parts, for it is protected by the relatively thick 

 root-cap. It is remarkable that although the radicle 

 bends away, when one side of the tip is slightly touched 



