THE PLANT BODY 35 



conclusion, given in a summary of his chapter on the " sensi- 

 tiveness of the radicle (root) to contact and other irritants," 

 indicates the nature of his contribution to the general subject 

 of the response of the root to various stimuli, including gravity, 

 in its natural growth through the soil. 



The peculiar form of sensitiveness which we are here considering 

 is confined to the tip of the radicle for a length of from 1 mm. to 

 1.5 mm. When this part is irritated by contact with any object, by 

 caustic, or by a thin slice being cut off, the upper adjoining part of 

 the radicle, for a length of from 6 or 7 to even 12 mm., is excited 

 to bend away from the side which has been irritated. Some influ- 

 ence must therefore be transmitted from the tip along the radicle 

 for this length. The curvature thus caused is generally symmetrical. 

 The part which bends most apparently coincides with that of the 

 most rapid growth. The tip and the basal part grow very slowly, 

 and they bend very little. 



Considering the several facts given in this chapter, we see that the 

 course followed by a root through the soil is governed by extraordi- 

 narily complex and diversified agencies, by geotropism acting in 

 a different manner on the primary, secondary, or tertiary radicles ; 

 by sensitiveness to contact, different in kind in the apex and in 

 the part immediately above the apex ; and apparently by sensitive- 

 ness 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 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 radi- 

 cles' 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 re- 

 side 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 com- 

 pared with a burrowing animal such as a mole, which wishes to 

 penetrate perpendicularly down into the ground. By continually 



