186 Parr .— The Response of Pilobolus to Light . 
A number of preliminary experiments were necessary to determine its 
behaviour under the normal laboratory conditions. It was found that the 
rapidity of sporangial development bears a direct relation to the tempera¬ 
ture. At 28° C. in the greenhouse sporangia are matured and projected 
early in the morning ; at 20° C. they are ejected at noon ; and at tempera¬ 
tures maintained below 8° C. they are matured and ejaculated after 
four days. 
It was noted that in direct light there is a tendency to earlier bulb 
formation than in the shadow, which is probably due to difference in 
transpiration (Lakon, 1907). Cultures which were well ventilated and kept 
in complete darkness developed sporangiophores of the usual length and 
ejected the sporangia but little later in the day than those grown in 
the same greenhouse in the light. Cultures kept in a laboratory where 
illuminating gas was used died out when windows were closed (cf. Crocker 
and Knight, 1908). In this series of experiments special care was taken to 
work in rooms free of gas. 
Cultures of Pilobolus were grown in cases in the greenhouse at 
a relative humidity of 90 per cent. They were transferred early in the 
afternoon, before any tips were visible, to trays of moist sand and covered 
with earthen jars furnished with bent-pipe ventilator tubes. Late in the 
afternoon a culture was taken from the tray, revolved for one minute 
in a measured spectral region to attune all parts equally (Pringsheim, 1912, 
p. 159 ; Clark, 1913^. 740), and then was exposed to one-sided illumination 
from this region for a definite period. During the exposure the culture 
rested on moist sand and was protected with a blackened shield to exclude 
stray light and to maintain the moisture conditions uniform. The moisture 
content of the air under the blackened shield as measured with Lambrecht’s 
polymeter ranged between 85 and 92 per cent, relative humidity. The 
culture after exposure was returned to the tray and covered with the 
ventilated jar. After one hour it was examined with a reading glass 
and the number of curved sporangiophores recorded. Only cultures having 
sporangiophores with pointed tips were used. Strict observance of the 
physiological state was found necessary since a culture with swollen 
tips exposed for three hours to the green rays failed to respond, while 
one with pointed tips responded in seventy minutes. The time of presenta¬ 
tion which was required to produce a curvature after a transmission time of 
one hour in approximately one-half the specimens in a culture, at the period 
of greatest sensitivity to stimuli, was taken as the standard for the measure¬ 
ment of the reaction of Pilobolus to light. No account was taken of 
reactions which occurred in extremely short intervals of time designated by 
Clark as ‘ first positive ’ curvatures. The longer intervals recorded in the 
following data without doubt correspond to those which he calls ‘ second 
positive * reactions. 
