99 BOTANY, 
of the root is sensitive to pressure, and when it comes in 
contact with any object bends from it. In this way the 
root-tip guides the advancing root through the interstices 
of the soil, avoiding on every hand the pebbles and harder 
bits of earth. The root-tip appears, also, to be sensitive 
to moisture, bending towards that side which is most moist, 
and thus in a dry soil the roots are constantly guided into 
those parts where the moisture is most favorable. 
189. Not only is the root-tip endowed with the power of 
circumnutation, but, in the words of Mr. Darwin, “ All the 
parts or organs in every plant whilst they continue to grow 
are continually circumnutating. If we look, for instance, 
at a great acacia-tree, we may feel assured that every one 
of the innumerable growing shoots is constantly describing 
small ellipses; as is each petiole, sub-petiole, and leaflet. 
The flower-peduncles are likewise continually circumnu- 
tating; and if we could look beneath the ground and our 
eyes had the power of a microscope, we should see the tip 
of each rootlet endeavoring to sweep small ellipses or cir- 
cles, as far as the pressure of the surrounding earth per- 
mitted. All this astonishing amount of movement has been 
going on year after year since the time when, as a seedling, 
the tree first emerged from the ground.” 
190. This general power of movement is subject to modi- 
fication by various agencies. Thus we find that in most 
plants the change from daylight to the darkness of ‘night 
is accompanied by changes of position in many parts, the 
nocturnal position being called the sleep of the plant. So, 
too, the influence of direct light produces a bending or 
turning of certain parts of plants toward the light, a kind 
of movement which has been called heliotropism. Gravi- 
tation has, also, been found to produce a special modifica- 
