THE MOVEMENTS AND SENSITIVITY OF PLANTS 325 



slowly on the klinostat, no curvatures will be formed, as by such 

 rotation the unilateral effect of the force of gravity is removed. 

 But if previous to setting the klinostat in motion, the seedling is 

 left for several minutes in a horizontal position, not so long, 

 however, as to prevent its beginning to bend, then after a certain 

 time it will form a curvature on the klinostat. The former upper 

 side of the stem will become concave, while the root will be convex. 

 In other words, the shoot will 

 bend in the same way as if it 

 were left motionless in a hori- 

 zontal position. 



This experiment clearly 

 proves, that in geotropism 

 there is in operation a phe- 

 nomenon of the same nature 

 as in phototropism. Owing to 

 the unilateral effect of the force 

 of gravity, a condition of 

 excitation is evidently pro- 

 duced in the plant, which is 

 not equally distributed be- 

 tween 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 which produced excita- 

 tion — the unilateral effect of 

 gravitation — was 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, but to 2 to 3 min., while the time of reaction requires 

 45 to 60 min., and sometimes even several hours. 



The force of gravity is quite uniform at every point on the 

 earth's surface. Therefore, it is impossible to reduce it to a certain 

 dosage, as we may the intensity of light in studies of phototropic 

 irritation. By changing the angle of deviation from the vertical 

 line, however, it is possible to modify the intensity of geotropic 



Fig. 133. — The rising of a cereal plant 



due to renewed growth and a curvature 



formed by the nodes (after Nathansohn). 



