it discriminated between the attached squares of card-like and thin paper.  Consequently it will tend to bend from the harder soil, and will thus follow the lines of least resistance.  So it will be if it meets with a stone or the root of another plant in the soil, as must incessantly occur.  If the tip were not sensitive, and if it did not excite the upper part of the root to bend away, whenever it encountered at right angles some obstacle in the ground, it would be liable [page 198] to be doubled up into a contorted mass.  But we have seen with radicles growing down inclined plates of glass, that as soon as the tip merely touched a slip of wood cemented across the plate, the whole terminal growing part curved away, so that the tip soon stood at right angles to its former direction; and thus it would be with an obstacle encountered in the ground, as far as the pressure of the surrounding soil would permit.  We can also understand why thick and strong radicles, like those of Aesculus, should be endowed with less sensitiveness than more delicate ones; for the former would be able by the force of their growth to overcome any slight obstacle.

After a radicle, which has been deflected by some stone or root from its natural downward course, reaches the edge of the obstacle, geotropism will direct it to grow again straight downward; but we know that geotropism acts with very little force, and here another excellent adaptation, as Sachs has remarked,* comes into play.  For the upper part of the radicle, a little above the apex, is, as we have seen, likewise sensitive; and this sensitiveness causes the radicle to bend like a tendril towards the touching object, so that as it rubs over the edge of an obstacle, it will bend downwards; and the curvature thus induced is abrupt, in which respect it differs from that caused by the irritation of one side of the tip.  This downward bending coincides with that due to geotropism, and both will cause the root to resume its original course.

As radicles perceive an excess of moisture in the air on one side and bend towards this side, we may infer that they will act in the same manner with respect to moisture in the earth.  The sensitiveness to moisture

* ‘Arbeiten Bot.  Inst., Würzburg,’ Heft iii. p. 456. [page 199]

resides in the tip, which determines the bending of the upper part.  This capacity perhaps partly accounts for the extent to which drain-pipes often become choked with roots.

Considering the several facts given in this chapter, we see that the course followed by a root through the soil is governed by extraordinarily complex and diversified agencies,—­by geotropism acting in a different manner on the primary, secondary, and tertiary radicles,—­by sensitiveness to contact, different in kind in the apex and in the part immediately above the apex, and apparently by sensitiveness to the varying dampness of different parts of the soil.  These several stimuli to movement are all