CHAP. III. SUMMARY OF CHAPTER. 199 



resides in the tip, which determines the bonding 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 ter- 

 tiary 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, when this acts obliquely 

 on a radicle, which has been deflected from its perpen- 

 dicular ' 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 being 

 highly sensitive to other stimuli, is of any service to 

 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 highly 

 important that the apex should pursue from the first 

 the most advantageous direction ; and we 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 bend either 

 from or to the exciting cause. A radicle may be 

 compared with a burrowing animal such as a mole, 

 which wishes to penetrate perpendicularly down into 

 the ground. By continually moving his head from 

 side to side, or circumnutating, he will feel any stone 



