resistance. The chief importance of this power of movement, however, comes from the way it may be modified, and its force augmented in certain directions by different influences.
Prominent among these influences is that of gravity. A most noticeable fact in the sprouting of seeds is that the root points toward the center of the earth, and the young shoot in the opposite direction, and it has long been known that this tendency to assume the vertical can not be explained as a response to differences in illumination, warmth, or moisture, since the organs be- have just the same when seedlings are grown under conditions where these differences are entirely eliminated. Moreover, if a root which has been growing downward be placed in a horizontal position, the region of growth, for a few millimetres behind the tip, will in the course of some hours bend so as to bring the tip into its original vertical position ; and as this bending will take place against an appreciable resistance, it follows that the assumption of the new position is not a mere drooping, but is a movement actively performed as if in response to a stimulus.
Fig. 2.—Circtmnutation of Radicle (Brassica) traced on horizontal glass from 9 a. m. January 31st, to 9 p. m. February 2d. Movement much magnified. (From Darwin's Power of Movement in Plants.)
Fig. 3.—Tracks left on Inclined Smoked Glass Plates by Tips of Radicles (Phaseolus) in growing downward. A and C, plates inclined at 60 ; B, inclined at 68° with the horizon. (From Darwin's Power of Movement in Plants.)
That gravity is the stimulus which evokes this response, was first proved by Knight in 1806. He reasoned that "as gravitation could produce these effects only while the seed remained at rest and in the same position relative to the attraction of the earth, ... its operation would become suspended by constant and rapid change of position of the germinating seed, and it might be counteracted by the agency of centrifugal force." He accordingly attached a number of germinating beans in various positions to
- On the Direction of the Radicle and Germen during the Vegetation of Seeds. Thomas Andrew Knight. Philosophical Transactions, vol. xcvi.