KENNETH V. THIMANN AND C.EORdE M. CURRY 667 



lie at the basal ciui of the cell and wlien the plant is turned throui^h 

 i)0'^ they fall on to the lateral wall. Vocum in 1958 (unpub.) showed 

 that the time taken in falling to the lateral side is about 2 minutes, 

 while the exposine time for threshold geotropic response is known 

 to be 2 to 4 minutes in the same plants. Now in shoots at least, 

 starch is produced only in plastids, and, as far as known, starch grains 

 are merely modified plastids. We thus arrive at the interesting con- 

 clusion that light and gravity are probably detected by similar 

 organelles. 



The movement of auxin from cell to cell is brought about by an 

 oxygen-dependent transjjort system which operates in a polar fashion, 

 i.e., auxin molecules move from the apex toward tlie base. This means 

 that tlie basal cross-wall, against which the statoliths lie, is the 

 fa\'ored point of export of auxin molecules, and the two facts may 

 be causally related, i.e., the contact of wall and particle may promote 

 auxin movement through the wall. When the plant is horizontal 

 and the statoliths rest against the lateral wall they now promote the 

 movement of auxin through this wall instead, thus causing lateral 

 translocation. 



It is now proposed that unilateral light would cause migration of 

 the plastids to the lateral wall opposite the light direction, thus 

 favoring the transport of auxin through that wall. 



Such a concept Avould bring phototropism into line with a hitherto 

 unrelated light response, the movement of chloroplasts. These move- 

 ments, ^vhich have been known and studied for a hundred years 

 since they were first recorded by Boehm (1857-9) and Borodin (1869) , 

 have so far only been observed on green chloroplasts, and it is not 

 yet known whether the proplastids of etiolated plants behave in the 

 same way. However, there are some suggestive parallelisms. In the 

 first place, plastid movements show a change of sign with increasing 

 light intensity: at low light dosages they move to the illuminated 

 Avails, while at high ones they revert to the walls perpendicular to 

 the light beam. In the second place, the low light response is sensi- 

 tive only to blue light and has a peak of effectiveness at about 450 m//, 

 (Zurzycka, 1951) . In the third place, proplastids are strongly affected 

 by red light treatment; the "primary granum" grows out rapidly 

 to form the well-known lamellae of the normal chloroplast, as has 

 been demonstrated by several electron microscopists (e.g., Miihle- 

 thaler and Frey-^Vyssling, 1959). This could give a possible basis 

 for the marked effect of red light in modifying phototropic sensitivity. 



