TROPISMS 175 



illumination on auxin curvatures has been carried out by- 

 van Overbeek (19366, 1936c). The effect depends upon the 

 auxin used. If indole- acetic acid is applied, light causes a 

 temporary decrease in rate of curvature, followed by an 

 increase, so that the net effect is small, while if auxin a is 

 used, light produces a considerable reduction in the curva- 

 ture. On the other hand, if the plants are illuminated for 

 2 hours before the auxin is applied, the curvatures produced 

 by indole-acetic acid and auxin a are both affected in the 

 same way, i.e. they are increased. Thus we must conclude 

 (1) that previous lighting increases the sensitivity of co- 

 leoptiles to both auxins, but that (2) lighting while the 

 auxin is present decreases the response to auxin a only. 

 The two effects could be interpreted as due to destruction 

 of auxin a inside the coleoptile in the light. Thus, in (1), the 

 increased sensitivity to applied auxin would be due to a 

 reduced auxin content of the coleoptile, while in (2) the 

 applied auxin a would be partially inactivated as soon as it 

 enters. We saw in VI C that light has no effect on auxin 

 transport, but on the other hand the auxin which is deter- 

 mined in transport experiments may be distinct from the 

 auxin in the cell (c/. Table XI, p. 132). Hence light may 

 inactivate only the auxin inside the cell, and not the trans- 

 portable auxin. 



Another effect of light is to cause the formation of auxin 

 in green parts, as discussed in Chapter IV A. Since the 

 greening of etiolated plants is itself brought about by hght, 

 this factor may also play a part in long-period illumina- 

 tions. 



The so-called euphotometric movements, or bending of 

 leaves out of the shade into the light (see e.g. Raydt, 1925), 

 are growth reactions of the petioles. These are apparently 

 due to a differential distribution of auxin in the petiole, 

 caused by differential illumination of various parts of the 

 leaf -blade {u). Laibach and Fischnich (1936a) have imi- 

 tated the movements by applying dots of auxin paste on 

 the leaf-blade of Coleus; if applied at one side the petiole 



