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 kina 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, 
