656 LIGHT AND LIFE 



just below the tip. Mrs. Sorokin, in my laboratory, has confirmed 

 these observations (Fig. 7) ; excellent crystals appear from 250 /x be- 

 low the tip down to about 2000 fx.; below this point, and in the 

 extreme tip, very few crystals could be seen. Thus the carotenoid is 

 localized close to the region of highest light-sensitivity; but again 

 the conclusion is not unequivocal, since the highest sensitivity, ac- 

 cording to Lange, is in the tip itself. 



Several workers have claimed that albino mutants of various seed- 

 lings contain little or no carotene and yet show "normal" photo- 

 tropic sensitivity. However, such albinos usually contain not less 

 than 5-10% of the carotenoid of the normal siblings, and there is 

 no information as to the absolute quantity of photoreceptor neces- 

 sary for phototropism. Furthermore, the claims of normal sensitivity 

 have not been supported by quantitative data in the range of pro- 

 portionality to light dosage, and usually rest on very long exposures. 

 In the case of the white Helianthus mutant of Wallace and Schwart- 

 ing (1954) , our own experiments show that its phototropic sensitivity 

 is indeed abnormally low. Galston and Asomaning (1960) have re- 

 cently experimented with an albino mutant of Colsess barley. Its 

 photosensitivity in the proportional range was about 30-50% of that 

 of the potentially green (but etiolated) form, while the data for total 

 carotenoid, mostly lutein, in the tip region of the two plants were 

 respectively 0.7 and 1.8, which is close to the ratio of the sensitivities. 

 The content of FMN, on the other hand, was the same in both. They 

 found also that both in oats and in barley preexposure to red light 

 about doubles the concentration of FMN; yet in oats the photo- 

 sensitivity is decreased, while in barley it is unchanged or even some- 

 what increased. 



On balance, these varied observations favor the carotenoid as 

 photoreceptor, and certainly it has been proved to be present, which 

 is not the case for methyl-lumiflavin. However, more work is called 

 for and the matter must be left there at present. 



Different Types of Phototropism in Avena 



The second of the major questions raised concerned the number 

 of different types of phototropic response. As far as the Avena coleop- 

 tile is concerned an answer to this question was given long ago, when 

 Arisz (1915) made careful measurements of the phototropic response 

 to a graded series of light dosages. These showed that while low 

 dosages cause a positive curvature, higher ones, if the time of ex- 

 posure is not too long, may give a negative curvature. Still more 



