DAR WIN ON MO VEMENT IN PLANTS. 25 



twining vines wind about other objects, bending successively to all points of the 

 compass, so that the tip revolves. Thus if the tip of such a plant at a certain 

 time bend toward the north, it will afterward " be found gradually to bend more 

 and more easterly, until it faces the east ; and so onward to the south, then 

 to the west, and back again to the north. If the movement had been per- 

 fectly regular, the apex would have described a circle — or, rather, as the stem is 

 always growing upward, a circular spiral." But the figure described is always 

 irregularly oval or elliptical, because this motion is never perfectly uniform for 

 all sides. 



This phenomenon of the revolution of the apices of plants is called by Mr. 

 Darwin '■'■ circumnutationr It is shown in the present work that circumnuta- 

 tion is not peculiar to twining plants, but that in a greater or less degree all grow- 

 ing parts of every plant — the roots, branchlets and leaves — have the same motion. 

 Moreover, the various movements of plants are nearly all simple modifications of 

 circumnutation. 



" The great sweeps made by the stems of twining plants and by the tendrils 

 of other climbers, result from a mere increase in the amplitude of the ordinary 

 movement of circumnutation. The position which young leaves and other organs 

 ultimately assume is acquired by the circumnutating movement being increased 

 in some one direction. The leaves of various plants are said to sleep at night, 

 and it will be seen that their blades then assume a vertical position, through modi- 

 fied circumnutation, in order to protect their upper surfaces from being chilled 

 through radiation. The movements of various organs to the light, or from it, 

 are all modified forms of circumnutation — as, again, are the equally prevalent 

 movements of stems, etc., toward the zenith and of roots toward the center of the 

 earth. 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 smab 

 ellipses; as is each petiole, sub-petiole and leaflet. * * If we could look be. 

 neath 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 circles, as far as the 

 pressure of the surrounding earth permitted. 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." 



The growth of young seedlings and the motions of the different parts of the 

 young plants are the subjects of a majority of the experiments made. The very 

 tip of the root is its most sensitive part, and it alone controls the direction taken 

 by the descending axis. Wherever it goes, the growth of the root must follow; 

 hence it is very important to the plant that from the first the root-tip should fol- 

 low the best possible path. The natural direction of the root is downward, fol- 

 lowing the impulse of gravitation {geotropism) ; the tip, if the soil permits, pur- 

 suing a spiral or corkscrew-like direction downward. The root-tip, moreover, is 

 sensitive to contact with different substances, and chooses the direction of least 

 resistance ; it is sensitive to moisture, turning generally in the direction of the 

 greatest dampness; and to the action of light, usually turning away from it. 



