SENSITIVENESS. 57 
marked characteristics, and this tendency to grow, or 
move towards the centre of the earth under the influ- 
ence of gravitation, is known as ‘‘ geotropism,” the op- 
posite tendency being called ‘‘apogeotropism.” Knight 
was the first to show that downward tendencies of the 
root were due to gravitation, and this he did by causing 
seedlings to grow on a wheel kept in motion. The effect 
of gravity was here overcome by the movement of the 
wheel, and the rootlets, instead of growing downwards, 
were now directed away from the centre of the wheel. 
Darwin shows (i. ¢., p. 540) that it is the tip of the root 
alone that is involved in this downward tendency, the 
destruction of the tip putting a stop to the movement. 
While the primary root or radicle under favorable cir- 
cumstances penetrates the soil perpendicularly downwards, 
the secondary ones bend obliquely, not perpendicularly, 
downwards, the tertiary ramifications and their subdi- 
visions being so little affected by geotropism that they 
grow out freely in all directions. From this manner of 
growth in the main root and its branches respectively, it 
is evident how the whole mass of soil within their reach 
becomes, under favorable conditions, a happy hunting 
ground for the roots. Moreover, it has been shown that 
where the primary radicle, the origin of the “‘ tap” root, 
has been destroyed—as it often must be in nature, by 
insects or other means—the secondary roots, instead of 
retaining their oblique direction, assume that previously 
taken by the injured root and pass downwards. 
The Action of Light and Heat on Roots.—The direct 
action of light upon ordinary roots is, of course, usually 
of a negative character. The form and direction of 
growth in the root may, however, be affected by differ- 
ences of temperature, experienced now on one side, now 
on another. Darwin has shown that the movements of 
roots, due to irritation or contact, are checked by too 
