i 82 
Parr .— The Response of Pilobolus to Light . 
tropism in Arena sativa, in which he found that response takes place 
according to the Talbot law only in the so-called ‘ first positive ’ reaction, 
and that it does not follow the Talbot law in negative or in the ‘ second 
positive J reaction. He made no claim to definite values of intensity 
(p. 739)> since he was unable to make exact photometric measurements. 
Jennings (1906) and Verworn (1913) attribute response to a change in 
the physiological state in the organism. The physiological state depends 
upon meta’bolic processes which are influenced by external factors. 
II. Statement of Problem. 
From a consideration of the literature it is evident that the chief 
reason for the unsettled condition in regard to heliotropic response lies 
in the lack of accurate measurements of the quantity and quality of the 
light. The writer, by carefully measuring and calibrating the quality and 
intensity of the light stimulus, undertook the difficult task of correlating, if 
possible, the conflicting results and views held with reference to the nature 
of the light stimulus. This was attempted (1) by a study of the response 
of Pilobolus to carefully calibrated light of different wave-lengths and 
intensities, and (2) by a determination of the energy relation, if any, between 
this light and heliotropic response. 
III. Materials and Methods. 
To determine the relative energy values in the different spectral 
regions it was necessary to employ very delicate physical instruments 
capable of calibration in standard units. A thermopile and galvanometer 
constructed by Professor W. W. Coblentz of the United States Bureau 
of Standards were used for this purpose. The thermopile was chosen 
because of its non-selective action in the different regions of the spectrum. 
The spectral regions used for experimental purposes were tested with 
a spectrometer from Adam Hilger and Co., London, and the limits of the 
wave-lengths determined. Cultures of Pilobolus , grown under controlled 
conditions and kept in absolute darkness for three hours preceding the 
formation of the sporangiophores, were exposed to these measured spectral 
regions, and the presentation time determined in each region. The thres¬ 
hold of stimulation thus found was taken as the measure of the sensitivity 
of Pilobolus to light of the different spectral regions. 
The apparatus was of the type quite generally used in spectral experi¬ 
ments where artificial light is the source of illumination (Yerkes, 1910, 
Laurens, 1911 ; Day, 19x1 ; Gross, 1913). Two light-tight compartments 
(a, Fig. 1), each 270 x 30 x 30 cm., were placed as wings at angles of 120° to 
a middle compartment (b), 120x30x30 cm. The inside was blackened 
throughout with lampblack and provided .with screens (c) to shut off 
