180 Parr .— The Response of Pilobolus to Light. 
and red rays when mixed with them. Using the sun’s spectrum, he 
obtained practically the same results. In both series of tests he found 
a first maximum between violet and ultra-violet, and a second between the 
red and infra-red. The effect decreased from either end of the spectrum to 
zero in the yellow. 
Dandeno (1903) with glass filters obtained results which differ widely 
from those of the investigators already mentioned. He found a first 
maximum in yellow and a second in blue, with the minimum in green. 
His screens, when spectroscopically tested, did not give pure colour. 
Sorokin (1873), Fischer von Waldheim (1872), and Brefeld (1881), 
studied the effect of light passed through a solution of potassium bi¬ 
chromate, and an ammoniacal solution of copper oxide, on Pilobolus , 
reporting very different results. Thus Sorokin claims that Pilobolus fails 
to grow in light filtered through the solution of copper oxide and that 
it gives a negative response to light filtered through potassium bichromate. 
Fischer von Waldheim obtained a strong positive response to the blue 
light, while Brefeld had a positive reaction in both blue and red, especially 
strong in the red. 
The divergent results of the investigators who have worked towards 
establishing a relation between refrangibility and response have been 
summarized by the author in Table I. 
Table I. 
Observer. 
Spectral regions. 
Plant. Red. Orange. Yellow. Green. Bhie. Indigo. Violet. 
Payer (1842) 
Cress 
0 
0 
0 
Zantedeschi (1842) 
Oxalis 
0 
0 
0 
Dutrochet (1844) 
Cress 
0 
0 
0 
Dutrochet (1844) 
Other 
seedlings 
0 
+ 
+ 
-Gardner (1844) 
Rape 
0 
+ 
+ 
Guillemin (1858) 
Cress 
+ 
++ 
+ 
Sachs (1864) 
Mustard 
0 
0 
0 
Muller (1872) 
Cress 
+ 
+ 
Sorokin (1873) 
Pilobolus 
— 
— 
Fischer von Wald¬ 
heim (1875) 
Pilobolus 
0 
0 
Brefeld (1881) 
Pilobolus 
+ 
+ 
Wiesner (1879) 
Vicia 
+ 
+ 
+ 
Grantz (1898) 
Pilobolus 
+ 
+ 
Dandeno (1903) 
Seedlings 
+ 
+ 
+ 
Blaauw (1909) 
Phyco- 
myces 
0 
0 
0 
0 
0 
++ 
+ 
+ 
0 
0 
+ 
+ 
+ 
+ 
0 
0 
+ 
+ 
+ 
0 
+ 
+ 
+ 
+ 
0 
+ 
+ 
+ 
+ 
+ 
0 
+ 
+ 
+ 
+ 
+ 
++ 
0 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
- 
0 
0 
0 
0 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
— 
+ 
+ 
+ 
++ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
++ 
+ 
+ 
+ 
+ 
+ 
+ 
+ 
++ 
+ 
The above summary shows the conflicting results obtained by a number of investigators on the 
response of plants to rays of light of different refrangibility. 
o = no response, + a positive, ++ maximum response, — negative response. 
4. The attempts to correlate the heliotropic response in plants with an 
energy value of the light appears first in the work of N. J. C. Muller (1872). 
While experimenting with cress seedlings in the objective spectrum, he found 
