ALBERT

Notes: The root cap of Lepidium sativum under our culture conditions was found to contain 7 (or occasionally 8) storeys of starch-containing cells. In the youngest one (or two) of these storeys the amyloplasts are small and the cells appear embryonic. In the 6 non-embryonic storeys the amyloplasts are large. Upon inversion of the root, the amyloplasts in the 3 youngest of the 6 non-emhryonic storeys start falling toward the opposite end of the cell at about 72 μ per h (at 21 C), hut they maintain this speed for only 6 to 12 min, after which they virtually come to a stop. As a result, it takes 10 to 12 min before any of the amyloplasts get approximately as close to the ceiling as they were to the floor before the inversion; and this is true only of the 2 youngest of the non-embryonic storeys. When the root is placed horizontal, whether coming from the normal or the inverted position, the amyloplasts reach the lower, longitudinal wall in 15 min or less. The positions of the amyloplasts in the cells of the 3 oldest starch-containing storeys are erratic and show little, if any, dependency on the preceding time of inversion.

Notes: Roots which are turned from their normal direction to directions at various angles with the plumb line develop the largest geotropic curvatures during a subsequent klinostat rotation period when the stimulation angle is well above the horizontal. In experiments with roots of Lepidium sativum L., the optimum is located at 120 to 140° when the stimulation time is between 2 and 15 min. If this fact is to be explained by the movements of amyloplasts in the root cap cells, one would expect roots which bad been kept inverted before the stimulation (so that the moveable amyloplasts are accumulated in the opposite end of the cells) to show an optimum angle well below 90°. — Pre-inversion of the roots did suppress the curvatures produced by stimulation at angles larger than 90° when measured after 10 to 30 min of klinostat rotation. This suppression may be taken as a support for the starch statolith hypothesis, since the amyloplasts in pre-inverted roots placed at angles exceeding 90° have a restricted opportunity to slide along the cell walls compared to non-inverted roots placed at the same angles. In pre-inverted roots measured after a period of klinostat rotation, however, no optimum was found at angles below 90°. When the stimulation time was 3.75 min, the response curves were nearly symmetrical about 90°. Stimulation for 15 min, on the other hand, resulted in curvatures which were much larger (although suppressed in comparison with non-inverted roots) when the stimulation angle was 165° than when it was 15°. During the 15 min stimulation period itself, however, pre-inverted roots curved 0.3° when stimulated at 15, but only 3.4° at 165°. This small difference was very highly significant and is in agreement with the starch statolith hypothesis insofar as the amyloplasts in pre-inverted roots placed at 15° have the greatest opportunity to slide along the cell walls. The lack of further development (and the actual decrease) of their curvatures during the subsequent klinostat rotation must then be due to other, depressing, factors, summarily designated as tonic. At angles above 90°, the tonic factors are either absent or even enhancing. Tbe tonic effects cannot be explained by amyloplast movements.