1920] 



HURD—FUCUS SPORES 



41 



was strong enough to cause all the rhizoids to turn sharply away 

 from it. Experiments were started, therefore, to determine 

 whether those behind the filters transmitting the longer wave 

 lengths would not also show this effect, the intensity of all the 

 lights being kept equal. The spores were germinated in small 

 dishes in darkness, and allowed to grow until the rhizoidal cell had 

 divided at least once. Cultures were then placed in the seven 

 dark boxes behind the original set of seven Wratten filters, and these 

 boxes placed at such distances from the naked arc that the inten- 

 sity of light behind each was 1804 meter-candles (table I). The 

 illumination was continued 6-7 hours. The next day the cultures 

 were examined to see which ones showed the characteristic negative 

 phototropism. It was found in every case that only those illumi- 

 nated by the blue and violet light had been so affected, those 

 behind the other filters having their rhizoids unbent, continuing 

 in the direction in which they had started, just as did the control 

 in darkness. With all the intensities the same (1804 meter- 

 candles), therefore, wave lengths capable of producing the negative 

 phototropism so commonly seen after white light illumination 

 are those of 4000-5200 Angstrom units, all the others having no 

 effect. 



TABLE III 



Filter no. 



70 



71 



72 



73 



74 



75 



76 



Control 



HICH WITH AN 

 NEGATIVE PHOTOTROPISM IN FllCUS RHIZOIDS 



OF 1804 METER-CANDLES PRODUCE A 



Color 



Red 



Red 



Orange 



Yellow 



Green 



Blue 



Violet 



White 



Wave lengths in 

 Angstrom units 





6600-7000 

 6200—6800 

 5900-6200 

 5600-5900 

 5 200-5600 

 4700—5200 

 4000-4700 



4ooo- L 7ooo 



Distance from arc 



in cm. at which 



intensities are 



equal 



80 

 68 



Appearance of 

 negative photo- 

 tropism 



57 



mm 



62 



— 



70 



mm 



62 



+ 



63 



+ 



85 



+ 



The same experiment was tried with sunlight as the source of 

 illumination. The young plants were exposed behind the filters 

 all day in a south window. The same results were obtained as 



