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Show 198 SUMMARY OF CHAPTER. CHAP. III. to be doubled up into a contorted mass. But we have seen with radicles growing down inclined phltes 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 JEsculus, should be endowed with less sensitiveness than more delica,te ones; for the former would Le able by the force of their growth to overcome any slight obstacle. After a radicle, which has been defieeted 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 tho touching object, so that as it rubs over the edge of an obstacle, it will bend downwarus; 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. A.s 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., Wurzbmg,' Heft iii. p. 45G. CHAP. III. SUMMARY OF CHAPTER. 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 com:se 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 more powerful than geotropism, 'vhen this acts obliquely on a radicle, which has been deflected from its perpendicular downward course. The roots, moreover, of most plants are excited by light to bend either to or from it; but as roots are not naturally exposed to the light it is doubtful whether this sensitiveness, which is perhaps only the indirect result of the radicles bein!r highly sensitive to other stimuli, is of any service t~ the plant. The direction which the apex takes at each successive period of the growth of a root, ultimately determines its whole course· it is therefore hio-hly . ' b 1mportant that the apex should pursue from the first the most advantageous direction; and wo can thus understand why sensitiveness to geotropism, to contact and to moisture, all reside in the tip, and why the tip determines the upper growing part to ben l either from or to the exciting cause. A. radicle may be co~pared with a burrowing animal such as a mole, wh1eh wishes to penetrate perpendicularly down into t~e ground. By continually moving his head from Slde to side, or circumnutating, he will feel any stone |