Geotropic responses in roots 



line. If the position of the plant were constantly adjusted in such a manner as 

 to maintain the root tip in the horizontal position, one might expect the 

 curvature to continue at the same constant rate for a considerable length of 

 time. 



It is clear from the upper curve in Figure 6 that the reaction time is surpris- 

 ingly short, at the most about 8 minutes. Definite curvatures could be 

 observed after 10 minutes. Whether the curve should be extrapolated back 

 as a straight line so as to cut the abscissa at 8 minutes, or whether it should 

 be extrapolated as a smooth curve approaching a point closer to the origin, 

 cannot be decided on the basis of these experiments. The latter procedure 

 has, however, been chosen in the present work. 



The approximately linear course of the curve is followed until the curva- 

 ture has reached a value of about 35°. At this point the sine of the angle of 

 attack of gravity is still large, about 0-91 . After 20 hours the mean curvature 

 is ca. 87°. 



The effect of klinostat rotation on the development of curvatures 



Plant chambers which were to be rotated were placed on the klinostat in 

 such a position that the slides and agar slices were initially vertical, and the 

 roots were parallel to the horizontal axis of the klinostat. Geotropic curva- 

 tures would thus develop in the plane between the agar slices. 



Rotation was begun after given periods of exposure {E) in the horizontal 

 position (mainly 0, 0-5, 4, 16, or 32 minutes). Curvatures were recorded 

 after rotation for various lengths of time {T). Only two rates of rotation 

 were used: 1 revolution in 0-5 minutes {RIO-5), and 1 revolution in 32 

 minutes (i?/32). The centrifugal forces developed by rotation were 

 1-3 X 10~^ Kg and 3-2 X 10^^ X^, respectively, i.e. they are entirely negligible 

 as compared with gravitation. 



Rotation at RjO-5 — The results of rotation at 1 revolution in 0-5 minutes are 

 shown in Figures 4 and 6. If roots are first exposed for 32 minutes in the 

 horizontal position and then rotated at 7?/0-5, they will have developed a 

 curvature of about 12° at the end of the exposure. They gradually enlarge 

 their curvature during the rotation until they reach a maximum value of 

 about 30° after some 100 minutes of rotation. The course of the curvature 

 after that time is rather uncertain because the variability increases as a 

 linear function of time, a consequence of the spontaneous movements (see 

 later). After an initial exposure of 16 minutes the curvatures follow a similar, 

 but lower course. If rotation is begun after 4 minutes of exposure, no 

 measurable curvature has developed at the start of the rotation. Otherwise, 

 the development of the response follows the same type of curve as in the 

 case of longer exposures. An exposure for 0-5 minutes yields positive mean 

 values, whereas the curvatures recorded during rotation without a preceding 

 exposure are, on average, practically zero. After about 32 minutes of 

 rotation, the curvatures in roots stimulated for 0-5 minutes are significantly 

 higher than in the rotated, unstimulated roots (cf. Figure 11). Since the 

 variability increases with time, a statistically significant difference could not 

 be demonstrated after longer periods of rotation using the present set of data 

 for exposures of and 0-5 minutes. Nevertheless, we may conclude that the 

 presentation time is shorter than 0-5 minutes. 



83 



