Mast, Light Reactions in Loiver Organisms. 163 



15. THRESHOLD. 



In ascertaining the threshold of photic stimulation for Volvox, 

 the colonies were put into a small glass aquarium constructed so 

 as to reduce reflection from the exposed surfaces as much as pos- 

 sible and thus avoid excessive variation in intensity. A description 

 of this aquarium w^as published in a preceding paper (Mast '06, 

 p. 386). The aquarium containing the colonies was then moved 

 from the source of light, a Nernst glower, until the light intensity 

 became so low that the organisms no longer responded to it. The 

 point at w^hich reaction ceased could, however, be only approxi- 

 mately ascertained, owing to marked individual variation in the 

 readiness with which they became acclimatized, to unavoidable 

 variation in the intensity of the source of light, and to the difficulty 

 of deciding, without the use of statistical methods, just where the 

 response to light ceased. But since the reaction of Volvox depends 

 quite as much upon its physiological condition as upon the inten- 

 sity of the light, it is evident that it is of no particular importance 

 to ascertain with great accuracy, either the threshold or the opti- 

 mum, unless the variations thereof can be correlated with the 

 physiological changes which cause them. We have no methods 

 of measuring the physiological condition of this organism with 

 any degree of accuracy, and therefore at present can hope to do 

 no more than study the effect of various stimuli on the threshold 

 and optimum. The follow^ing observations were made with the 

 view of ascertaining the general effect of exposure to light on the 

 variation in the threshold and optimum. 



On July 30, 1904, at 5 p. m., it was found that Volvox which 

 had been collected at 6 a. m. and kept in the dark all day responded 

 definitely to light of 0.16 candle meters'intensity, and rather defi- 

 nitely to light of 0.14 candle meters. This is the lowest intensity 

 to which any response was obtained at any time. Specimens 

 collected shortly after 12 m., July 14 and 15, respectively, and 

 tested as soon as brought into the laboratory responded to light 

 of 0.50 to 0.83 candle meters. The sky was clear on both of these 

 days, but the organisms were found among the water plants in 

 more or less shaded places. 



It was found at different times that after being exposed to direct 

 sunlight a few moments the colonies did not respond even to an 

 intensity as high as 500 candle meters. We have thus observed 

 the threshold to vary from 0.14 to 500 candle meters, and this 



