60 PLANT LIFE ON THE FARM. 



but that it induces movement in adjoining parts, on 

 which account the parts so influenced are spoken of as 

 "sensitive." 



Passage of Roots through the Soil Summary. The 



course followed by a root through the soil is, says Dar- 

 win, " brought about and modified by extraordinarily 

 complex and diversified agencies by geotropism, acting, 

 as has just been explained, in a different manner on the 

 primary, secondary, and tertiary radicles ; by sensitive- 

 ness 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. . . . 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 im- 

 portant that the apex should pursue from the first the 

 most advantageous direction ; and we can thus under- 

 stand why sensitiveness to gravitation, to contact, and to 

 moisture, all reside in the tip, and why the tip determines 

 the upper growing part to bend either to or from the 

 exciting cause. A radicle may be compared with a bur- 

 rowing 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 

 feels any stone or other obstacle, as well as any difference 

 in the hardness of the soil, and he will turn from that 

 side. If the earth is damper on one than on the other 

 side, he will turn thither as to better hunting ground. 

 Nevertheless, after each interruption, guided by the 

 sense of gravity, he will be able to recover his downward 

 course and to burrow to a greater depth." 



Elsewhere Darwin sums up the root movements as fol- 

 lows : " We believe that there is no structure in plants 

 more wonderful, so far as its functions are concerned, 

 than the tip of the radicle. If the tip be lightly pressed, 



