92 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 flrst 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 \^'hich has been called heliotropism. Gravi- 

 tation has, also, been found to produce a special modifica- 



