1076 



BIOLOGICAL EFFECTS OF RADIATION 



LIGHT INTENSITY 



The work of numerous investigators shows, as does that of Dillewijn, 

 previously mentioned, that Ught within a certain range of intensity 

 brings about an increased growth rate while that of another range results 

 in a diminished rate of growth. Recently WiechuUa (45) has carried out 

 a number of experiments on the sporangiophores of Phycomyces using 

 different amounts of light quantities expressed as meter-candle-second 

 (mks = Mcs) units. These results are graphically shown in Fig. 2. The 

 abscissa is time in minutes after illumination, and the ordinate, relative 



growth units, the growth rate in dark- 

 ness being unity. It is interesting to 

 note the character of these curves. 

 For the greater units, the reaction time 

 is short and the rate of growth rapid, 

 which later becomes less than the rate 

 in darkness. For the weakest light no 

 rate lower than that in darkness is 

 noted. Also the weaker the light the 

 longer the reaction time. 



Castle (14) states: "Alleged reversal 

 of the phototropism of the sporangi- 

 ophores of Phycomyces by high inten- 

 sities of light does not occur if infra-red 

 radiation is properly excluded. Photo- 

 tropic 'indifference' alone occurs at 

 high intensities due to equal photic 



0.5 f 







20 

 1.5 

 1.0 

 0.5 





 20 

 1.5 

 1.0 

 0.5 





 2.0- 

 1.5- 

 1.0 

 0.5 





 2.0- 

 1.5- 

 t.O 

 0.5 







40 MKS 



10 MKS 



6 MKS 



MKS 



4mks 



10 15 20 25 30 



Fig. 2. — Graphs showing growth 



reponse of sporangiophores of Phyco- action On both sideS of the Sporaugi- 



myces for different light quantities , rr l ^ j • j. • • j. 



indicated in meter-candle-seconds. The ophore. It heat radiation IS not 



growth rate in darkness is taken as screened out, a gradual, negative 



unity. (From WiechuUa, 45.) ., . • i i- - i i >> 



thcrmotropic bending takes place. 

 Castle (15) has shown that phototropic "indifference" results from 

 a failure of light to bring about differences in the rates of growth on 

 the two sides of the sporangiophore. By reducing the exposure time 

 a critical light duration period is found below which sensitive indifferent 

 sporangiophores will show phototropic bending. He therefore concludes 

 that phototropic bending occurs when the illumination on one side is 

 submaximal and that indifference results when equal and maximal 

 photochemical action takes place on both sides of the sporangiophore. 



WAVE-LENGTH 



In many of the early experiments on phototropism it is impossible to 

 distinguish the wave-length effects from those of intensity. Parr (33) 

 studied the response of Pilobolus to different wave-lengths and intensities 

 of light which were carefully measured. The sources used were a Nernst 



