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 geotropisin, 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. Bya 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 
