GROWTH OF PLANTS 147 



as to cause it to bend, then after a certain time it will form a 

 curvature on the clinostat. The original upper side of the stem 

 will become concave, while the original upper side of the root will 

 become convex. In other words, the stem and root will bend 

 in the same way as if they had been left motionless in a horizontal 

 position. 



This experiment clearly proves that owing to the unilateral 

 effect of the force of gravity, there is produced in the plant a 

 condition of excitation that is not the same on its upper and 

 lower sides. After a certain period of time, this leads to a definite 

 reaction, the formation of a curvature, in spite of the fact that 

 during this period the cause that produced excitation, the uni- 

 lateral effect of gravitation, w^as removed by means of slow 

 rotation. The least duration of stimulation necessary to produce 

 a visible curvature is called the "presentation time." Usually 

 it is very short, amounting in the most sensitive plants, like the 

 stem of the sunflower, to 2 to 3 min., while the time for reaction 

 requires 45 to 60 min. and sometimes even several hours. j 



The force of gravity is nearly uniform at the earth's surface. 

 By changing the angle of deviation from the vertical line, how- 

 ever, it is possible to modify the intensity of geotropic stimu- 

 lation, which appears to be in proportion to the sine of the angle 

 of deviation from the vertical. This is known as the *4aw of 

 the sine." Therefore, the time of presentation seems to be least 

 in a horizontal position and increases with the gradual approach 

 to the vertical line. The product obtained by multiplying the 

 time of presentation by the sine of the angle of deviation is a 

 constant quantity. 



This constant indicates that the realization of the geotropical 

 reaction requires a definite amount of stimulus, there being no 

 difference between the prolonged action of a weak stimulus and 

 action for a short time of a strong stimulus. This relation has 

 been summarized as the law of the quantity of stimulus. This 

 law holds good not only for the phenomenon of geotropism, but 

 likewise for phototropism and other movements of the plant, as 

 well as for the phenomena of excitation in the animal organism. 



By changing the angle of deviation, it is possible only to 

 decrease the strength of the geotropic stimulus. In order to 

 increase it, it is necessary to apply the effect of a different force, 

 which also produces an acceleration of mass, viz., centrifugal 



