648 LIGHT AND LIFE 



parable but simpler experiments by Boysen Jensen, in which only 

 the curvature of slit coleoptiles was observed. Because it is of funda- 

 mental importance it will be referred to again below. 



We may conclude, then, that the ultimate effector of phototropic 

 curvature is a lateral diversion of auxin away from the lighted side 

 of the plant. This is, in effect, the present state of the problem. The 

 questions to be answered are of three types: 



(a) Is there more than one kind of phototropic response? 



(b) What are the photoreceptors? 



(c) What kinds of processes occur between the reception of the 

 light and the resulting asymmetric distribution of the auxin? 



Before presenting the evidence which helps to answ^er these ques- 

 tions, it should be stated that the auxin has been identified, at least 

 in the majority of plants, as indole acetic acid. It is well to mention 

 also that although in the earlier work several different grasses have 

 been used, our own experiments have been carried out only with 

 the oat (Avena) which is apparently about the most sensitive member 

 of the group, and with the fungus Phycomyces. It will be convenient 

 to take up the second question first. 



Nature of the Photoreceptor in Avena 

 Since phototropism is a growth response, which therefore requires 

 intact cells, the most direct route to the study of the photoreceptor 

 is through the spectral sensitivity of the response. Such an action 

 spectrum should indicate the absorption spectrum of the photorecep- 

 tor, and, if obtained in sufficient detail, might serve to identify that 

 compound. 



Several studies of spectral sensitivity have been made in the past, 

 but with insufficient detail for the drawing of reliable conclusions. 

 All agree on one point, nevertheless: curvature is not brought about 

 by red light. Our studies have shown, however, that oat seedlings 

 are by no means insensitive to red light. Previous exposure of the 

 dark-grown plants to red light of wavelength greater than 660 m^u, 

 changes their sensitivity to a subsequent dose of blue or white light. 

 What happens is that the distribution of growth rate along the 

 seedling is changed as shown in Fig. 2. The dark-grown plants 

 show two maxima of growth rate, one at about 6 mm below the tip, 

 and a second one around the node which separates the coleoptile 

 from the mesocotyl below. The effect of one hour's exposure to red 

 is very greatly to increase the growth rate in the zone 4-8 mm below 

 the tip and to suppress almost wholly the elongation at and below 

 the node. This finding made it necessary to control rigidly the ex- 



