40 RESEARCHES ON FUNGI 



612 (orange) ; 589 and 585 (yellow) ; 540 and 525 (green) ; 496 and 

 470 (blue) ; 465 and 438 (indigo) ; 414 and 398 (violet). 



Miss Parr's investigation, carried out with so much care and 

 precision, yielded a series of important results (Fig. 14), some of 

 which may be summarised as follows. (1) Pilobolus responds 

 heUotropically to the hght of all the regions of the visible spectrum 

 from red to violet. (2) The presentation time required to produce 

 a heliotropic curvature decreases gradually from red to violet or, 

 conversely, the hehotropic response of the fungus increases gradually 

 from red to violet. The minimum response is in the red and the 

 maximum in the blue. There are no intermediate maxima or minima 

 such as others have supposed to exist. (3) The presentation time 

 does not vary in direct ratio with the measured value of the energy 

 of the Ught in the different regions of the spectrum. (4) The 

 presentation time varies in inverse ratio to the square roots of the 

 wave frequency. (5) While light energy is a factor in the relative 

 time required for hehotropic excitation, yet the quality of the hght, 

 i.e. the frequency of the waves, is of more importance. (6) The 

 divergent views held by previous investigators regarding the spectral 

 region of maximum response may be explained on the basis of the 

 energy value and frequency of the light employed.^ 



Allen and Jolivette's Investigations. — A series of interesting 

 experiments upon the reactions of Pilobolus to light were described 

 by Allen and Jolivette ^ in 1914. They studied the accuracy of 

 aim of the fungus when influenced by (1) a single source of white 

 light, (2) two sources of white light used simultaneously or suc- 

 cessively, and (3) two sources of hght differing in colour. 



1 Miss Parr {loc. cit., p. 203) summarised her results in seven paragraphs, the first 

 four of which are substantially identical with (1) to (4) as given above. Her other 

 paragraphs were as follows. (5) The product of the square root of the frequency 

 times the presentation time decreases with the decrease in the energy value of the 

 spectral regions, and is an approximate constant for a given light source. (6) The 

 spectral energy in its relation to the presentation time may be expressed approxi- 

 mately in the Weber-Fechner formula, if the wave-frequencies be made a function 

 of the constant. (7) The relation of the spectral energy to the presentation time 

 may also be approximately expressed in the Trondle formula, the wave-frequencies 

 being a function of the constant. 



2 Ruth F. Allen and Hally D. M. Jolivette, " A Study of the Light Reactions of 

 Pilobolus," Trans. Wis. Acad., Vol. XVII, 1914, pp. 533-598. 



