INFRARED 287 



mediately after "short ultraviolet" exposure. There is some difference 

 of opinion in regard to the most effective wave length for different or- 

 ganisms or even different strains in one species of organism [Kelner (15), 

 Knowles and Taylor (16)]. No significant photoreactivation has been 

 found after exposure to x-rays. It appears from the present informa- 

 tion that photoreactivation either destroys a toxic substance which is 

 formed by irradiation with 2537 A or reverses the destructive action oi 

 an essential compound needed for the survival of the cell. 



The fact that photoreactivation is found after exposure to short ulti-a- 

 violet fits into the general picture of the behavior of cells after irradi- 

 ation with ultraviolet as compared with x-rays. Bacteria or fungi sur- 

 viving ultraviolet have a much extended lag phase [Hollaender and 

 Duggar (10)]. This extended lag phase has been determined very care- 

 fully, but it can also be recognized by observing plate cultures which 

 have been incubated about 12 hr. There is very little of this extension 

 after x-ray exposure. Our interpretation at present is that the extension 

 of the lag phase is a non-chromosomal effect and that photoreactivation 

 works mostly through the cytoplasm. Such an interpretation seems 

 reasonable at first but would have to be checked experimentally. 



Infrared 



We reported several years ago, first in cooperation with Kaufmann 

 (12) and later with Swanson (19), that infrared around 10,000 A given 

 before {Drosophila, Tradescantia, and Aspergillus terreus) or after {Trades- 

 cantia) x-radiation will increase the effectiveness of x-radiation in pro- 

 ducing chromosomal rearrangements and chromatid breaks and muta- 

 tions (Aspergillus terreus). The effect is somewhat more pronounced in 

 regard to chromatid breaks, as Giles and Beatty (unpublished) have 

 found in our laboratory. The infrared alone, given under carefully con- 

 trolled conditions, will produce no recognizable chromosome changes. It 

 is important to point out that a carefully designed experimental tech- 

 nique must be used so as not to raise the temperature in these biological 

 materials to a level in which heat damage could appear. This infrared 

 work indicates that, in addition to the usually recognized damage, some 

 "potential" damage must be produced in the chromosomes by x-radia- 

 tion which is not obvious under normal conditions and which can be re- 

 paired if x-radiation is given alone. These additional effects of infrared 

 radiations have also been observed in regard to nitrogen mustards by 

 Swanson and by Kaufmann. These experiments indicate that x-radia- 

 tion causes an unstable condition in chromosomes which can be made 

 obvious by infrared treatment. 



