Parr .— The Response of Pilobolus to Light. 185 
The mechanical value of radiation from the Hefner lamp burning 
o 
under standard conditions as determined by Angstrom is 20*6 x io -8 
sec cm ^ 
or about 8*3 ergs (Nichols, 1905) per second per square centi¬ 
metre at a distance of 1 metre (cf. Kniep and Minder, 1909). At 
2 metres the radiation has one-fourth of this value, or 2*075 ergs. Accord¬ 
ingly, whatever the sensitiveness of the galvanometer, the deflexion 
produced by the Hefner lamp under the given conditions is equivalent 
to 2*075 ergs. The energy values of the spectral regions used in the 
several experiments were measured and expressed as above indicated. 
Since a galvanometer of the delicacy of the one used is subject to 
disturbance, its sensitivity was frequently tested, and if necessary it was 
readjusted by a slight change in the position of the controlling magnet (cf. 
Coblentz, 1911). To prevent the air being vitiated by a too frequent use 
of the Hefner lamp, the red, yellow, and green regions of the spectrum were 
very accurately determined in their relation to the Hefner lamp standard, 
and these regions were then used as standards of comparison for other 
regions of the spectrum. It is evident that any region having its energy 
value once determined may serve as such a standard. The energy values 
of the opposite ends of the spectrum, which are so widely different, were 
thus measured with the galvanometer adjusted in sensitivity to the region 
in which the experiments were carried on. The values in every case, 
as shown in Tables IV and V, are expressed in terms of the Hefner lamp 
values and are directly comparable to each other. 
In the earlier work with the galvanometer the average of a series 
of ten deflexions was taken in each spectral region. It was subsequently 
found that three or four readings would give practically the same result, 
and, moreover, there was less chance for error arising from the change of 
magnetic and temperature conditions. 
Pilobolus. 
Pilobolus grows abundantly on manure obtained from the University 
dairy barns. Coemans (1859), Klein (1872), Brefeld (1881), and others 
have described its culture and mode of growth. The sporangiophores late 
in the afternoon appear first at right angles to the substratum, due to 
a negative hydrotropism, and later grow vertical, due to negative geotropism 
(Pfeffer, 1906). At this stage the sporangiophores are extremely sensitive 
to light stimuli. In the evening the tips gradually grow into spherical 
yellow knobs, and the sensitivity of the sporangiophore is very much 
decreased (Jolivette, 1914). During the night the sporangiophores become 
distended just below the yellow knobs, and the characteristic transparent 
bulbs are formed. In the morning the matured sporangia are projected 
with considerable force. 
