202 
Parr .— The Response of Pilobolus to Light . 
In Blaauw’s experiments (1909), because of the higher absolute tempera¬ 
tures of the light sources (sun and arc-lamp), the maximum energy is near 
the yellow. He obtained no response in the red end beyond the yellow- 
orange. In the indigo with a higher frequency and lower energy value 
he found a maximum response. This again accords beautifully with the 
results already described in this paper. 
Divergent views held by previous investigators regarding the region 
of maximum response in the spectrum can be readily explained on the 
basis of energy value and frequency. That this has not previously been 
done is largely to be attributed to the difficulty encountered in deter¬ 
mining the spectral values. Thus, Gardner (1844) observed that the 
intensity of light had only a subordinate influence. Sachs (1867) and Loeb 
(1906) state that the shorter waves are the more active and that the reaction 
is proportional to the intensity. Towle (1900) says of Cypredopsis that the 
response is shorter in the stronger light, ‘though the difference is too slight 
to warrant one in drawing any inferences from it’. Allen and Jolivette 
( I 9 I 3, p. 58 j) concluded from their experiments where coloured glass screens 
were used that ‘ the light of short wave-length has no preponderating 
influence at least in determining the phototropic reactions of Pilobolus \ 
It is noted in the experiment upon which this conclusion is based that the 
blue rays are balanced against the sum total of all the rays included in 
white light. From the relatively greater influence which the preceding 
pages of this paper have shown the frequency of light to exert upon 
Pilobolus , as compared to that of intensity, this conclusion from unmeasured 
quantities of light could be anticipated. 
In a later paper, however, Miss Jolivette (1914, p. 119), using different 
kinds of light sources, states, ‘ Pilobolus fires its sporangia in larger numbers 
towards the lights in which the blue rays are greatest. In other words, 
it is more responsive to actinic rays. The intensities in the different wave¬ 
lengths are not measurable.’ She further says that the energy given off 
by the source of light apparently does not compare in effect with the 
distribution of the same in different portions of the spectrum. In the 
experiments using a 16-candle-power tungsten and a 32-candle-power 
carbon filament lamp the large majority of the sporangia went to the 
tungsten light, although its total energy was but half that of the carbon. 
From this she concludes that the differences in distribution in the spectrum 
outweigh in effect the differences in the total energy of the two sources, 
a qualitative conclusion which the present paper through quantitative 
methods has shown to be valid. 
The writer will not endeavour to explain the physiological significance 
of either the Fechner or the Trondle formula in relation to the reaction 
of Pilobolus to different quantities of light energy. That there is a physio¬ 
logical principle within the living protoplasm expressed in "these formulae 
