201 
Parr —The Response of Pilobolus to Light. 
while the sun, at a probable temperature of 6,ooo° C., emits the greatest 
amount of energy in the violet (cf. Duff, p. 455). 
The Nernst and the tungsten lamps, heated to about 2,300° C. (Hyde), 
show the spectral energy to increase from the ultra-violet to a maximum 
in the infra-red (Moll, 1907; Coblentz, 1911). Since we find the energy 
highest in the red of the visible spectrum, we should expect from the 
discussion already given that in the response of Pilobolus , the slowness of 
the waves in the red region is in a measure compensated by their greater 
energy. The frequency having a much more noticeable effect than the 
energy values of the waves in the response of Pilobolus (Table IX, 
pigs, g and 4) may more than compensate for the lack of energy in 
the violet region. The regions between the red and violet having a gradual 
decrease in energy and an increase in frequency produce a response inter¬ 
mediate in time between that of the red and the violet. From the physical 
basis one could then predict the first maximum in ultra-violet and a second 
maximum in the orange in the response of plants exposed to a light having 
its maximum spectral energy in the orange, as the energy of this region 
would more than compensate for the slowness of the waves and a second 
maximum thus appear. 
The results obtained by Guillemin (1857), who by the use of different 
prisms was able to obtain spectra from the sun in which the relative 
intensity varied for the same wave-length, substantiate this view. With 
a rock-salt prism, which transmits dark heat rays, he obtained the first 
maximum in the ultra-violet and a second maximum in the ultra-red ; 
with a quartz prism, which transmits the chemical rays, he obtained the first 
maximum in the violet and a second between the red and infra-red; with 
a flint-glass prism, which best transmits the intermediate rays, he obtained 
the first maximum in the violet and a second in the green. He further 
found that the second maximum advanced more and more into the visible 
spectrum as the water vapour in the air was increased and the position of 
the sun approached the horizon. 
Wiesner’s results with seedlings in the sun’s spectrum may in a measure 
be correlated with this line of thought. Due to the vapour, &c., present in 
the atmosphere, the maximum energy of the sun’s spectrum at noon is 
usually near the yellow (Langley, 1883 , p. 33). The use of a biconvex lens 
would so focus these rays as to bring the intensity in the yellow to a very 
high value which may be responsible for the indifferent, or even negative 
heliotropic response (Oltmanns, 1892; Pringsheim, 1907; Blaauw,i909; Clark, 
1913). His results show a very regular decrease in the presentation time 
from the green into the ultra-violet, and bring the maximum irritability into 
the ultra-violet region. This would naturally follow from the increasing 
frequency and very considerable energy of this region, in the same manner 
as determined and recorded in this paper (Table IX). 
