34 THE POPULAR SCIENCE MONTHLY, 



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 oppo- 

 site direction, and it has long been known that this tendency to 

 assume the vertical can not be explained as a response to differ- 

 ences 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 as- 

 sumption of the new position is not a mere drooping, but is a 

 movement actively performed as if in response to a stimulus. 



I 



Fig. 2. Fig. 3. 



Fig. 2. Circtmnutation of Radicle (Brassica) traced on horizontal glass from 9 a. m. 



January 31st, to 9 p. it. 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 Ead t cles (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 180G.* 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 coun- 

 teracted 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. 



