38 RESEARCHES ON FUNGI 



divergent results which had been obtained by others in their studies 

 of the heUotropic response in plants lay in the lack of accurate 

 measurements of the quantity and quahty of the light employed, 

 undertook the task of attempting to correlate all previous results 



(1) by studying the heliotropic response of Pilobolus to carefully 

 caUbrated hght of different wave-lengths and intensities, and 



(2) by a determination of the energy relation, if any, between this 

 Ught and hehotropic response. 



In carrying out her work. Miss Parr made use of very dehcate 

 physical instruments which could be calibrated in standard units. 

 The hght sources were two, a Nernst lamp of single glower type and 

 a nitrogen-filled tungsten Mazda lamp. A beam of hght from one 

 of these lamps was broken up by means of a lens and prism. The 

 relative energy values of the hght in the different spectral regions 

 employed expressed in ergs per sec. per square cm. were determined 

 with the aid of a thermopile and galvanometer, the standard hght 

 energy used for comparison being a Hafner lamp kept at a distance 

 of 2 metres from the thermocouple (2-075 ergs). The spectral 

 regions used in the experiments were tested with a spectrometer 

 and the limits of the wave-lengths were thus obtained. Cultures 

 of Pilobolus, grown in coal-gas-free air under known conditions 

 of temperature, humichty, ventilation, etc., were kept in absolute 

 darkness for three hours preceding the formation of the sporangio- 

 phores. The sporangiophores, whilst still pointed and showing no 

 trace of the formation of sporangia at their ends, were then exposed 

 to measured spectral regions for a definite period of time which 

 varied from about 50 to 100 minutes (the presentation time). Each 

 culture, after exposure, was again placed in the dark and kept there 

 for an hour (the transmission time). Then with a reacUng glass 

 the number of curved sporangiophores was recorded. The presenta- 

 tion time which was required, before a transmission time of an 

 hour, to produce a curvature in approximately one-half of the 

 cultures was taken as the standard for the reaction of Pilobolus to 

 hght. To obtain this presentation time with a wave-length of any 

 one frequency it was necessary to make several experiments ; for, 

 with too short an exposure, none of the sporangiophores showed 

 any curvature while, with too long an exposure, all the sporangio- 



