326 Life and Immortality. 



observed that as soon as the tip merely touched a slip of 

 wood cemented across the plate, the entire terminal growing 

 point curved away, so that the tip soon stood at right angles 

 to its former direction ; and thus, as far as the pressure of 

 the surrounding soil would permit, would it be with an 

 obstacle encountered in the ground. Thick and strong rad- 

 icles, like those of the horse-chestnut, are endowed with less 

 sensitiveness than more delicate ones, and would therefore 

 be the better able by the force of their growth to overcome 

 any slight impediment to their progress. Further, as radi- 

 cles perceive an excess of moisture in the air on one side and 

 bend towards this side, it is reasonable to infer that they 

 will act in a similar manner with respect to moisture in the 

 earth, for the sensitiveness of moisture resides in the tip, 

 which determines the bending of the upper part. May not 

 this capacity partly account for the extent to which drain- 

 pipes often become choked with roots ? The direction which 

 the apex takes at each successive period of the growth of a 

 root, ultimately determines its whole course. It is therefore 

 very important that the apex should follow from the first the 

 most advantageous direction. We can thus understand why 

 sensitiveness to geotropism, contact and moisture should all 

 reside in the tip, and why it should determine the upper 

 growing part to bend either from or to the exciting cause. 

 Darwin has compared a radicle with a burrowing animal, 

 such as a mole, which wishes to penetrate vertically into the 

 ground. By a process of circumnutation, or the movement of 

 his head from side to side, he is enabled to feel any stone or 

 other obstacle, as well as any difference in hardness of soil 

 that may exist, and will therefore turn from that side; but if 

 damper on one side than on the other, will turn thither as a 

 more suitable hunting-ground. Nevertheless, after each inter- 

 ruption, he, guided by the sense of gravity, will be able to re- 

 cover his downward direction and to reach to a greater depth. 

 Destruction of the tip of a radicle does not prevent the 

 adjoining part from bending, if this part has already received 



