Geotropic responses in roots 



velocity of geotropic curvature at the end of the horizontal exposure. This is 

 the reason for assuming an accelerated course of the lower portion of the 

 curve for continuous exposure. It is not certain, however, whether the course 

 of the curves in Figure 7 reflects the velocity of the actual curvature or the 

 velocity of some limiting process controlling the rate of geotropic bending, 

 a process which is accelerated from almost the beginning of exposure, but 

 which reaches a constant rate after 16 minutes, or perhaps sooner. 



It may now be possible to estimate the length of the presentation time by 

 plotting the angles of (C^_^y— Cj,.) {Figure 7) after various times of rotation 

 and extrapolating the resulting curves. In an attempt to do this, the angles 

 at T = 32, 64, and 96 minutes were plotted against the logarithm of E 

 (see Figure 8). Extrapolation yields a presentation time of 0-3 minutes. 



■i E-32m\n 

 + £ =16 min 

 ° E-V- min 

 o E =0 min 



V \ / \ /\V/ \/ w \ / \ A 



m 2w 



Time — ► 



mm 



Figure 9. Development of geotropic curvatures during rotation at 1 revolution per 32 minutes (/?/32). 

 Ordinate: total curvature (C'^+y)- Abscissa: time (T) after begirming of rotation. Duration of 

 preceding horizontal exposure {E) indicated. Curvature at the end of exposure can be read at T = 0. 

 Symbols indicate actual determinations. Other maxima and minima were interpolated. 



The procedure applied here was chosen arbitrarily and other methods of 

 graphic presentation are possible. At any rate, however, the true presenta- 

 tion time must be somewhat shorter than 0-5 minutes, though it is probably 

 of this order of magnitude. 



Rotation at /?/32 — If unstimulated roots are rotated at /?/32, the root tips 

 bend back and forth with an amplitude very close to 5° {Figure 9). This 

 result was to be expected on the basis of the known velocity of curvature 

 during continuous exposure to 1 g and the size of the opposing stimuli 

 received during the rotation. Measurements were made at 16-minute 

 intervals, approximately when maxima and minima were to be expected. 



It was mentioned in a previous publication (Larsen, 1953) that pre- 

 stimulated roots tend to straighten out again if they are rotated slowly, 

 whereas their curvatures will increase if the plants are rotated at a higher 

 velocity. This has been confirmed in the present experiments {Figure 5). 

 The roots do not straighten out simply by curving in the direction opposite 

 to the initial one; they follow the motion of the klinostat just as unstimulated 

 roots do. The spontaneous movements which develop during rotation at 

 /?/0-5 seem to occur only in the plane of the interface between the two layers 



85 



