24 BOOKS AND CURRENT LITERATURE 



ous application under conditions of high humidity of so small a concen- 

 tration as 0.5 part of sulphur dioxide per million of air. Under ordi- 

 nary humidity conditions 20 parts per million produced severe injury 

 in 15 minutes; 10 parts per million produced injury in 30 minutes; 

 and 5 parts per million produced injury in an hour. Injury could 

 be produced by 2 parts per million applied for 4 hours continuously 

 or for 10 minutes daily during the entire growing season (10 to 12 weeks). 

 With 1 part per million slight visible injury was produced by two 7-hour 

 fumigations but a total of six 7-hour fumigations or ninety 10-minute 

 fumigations did not decrease the yield. 



The concentrations found by the Commission to be injurious were 

 surprisingly low and far below the concentrations reported by previous 

 investigators as the lowest capable of producing injury. This dis- 

 crepancy is explained as due to the fact that previous investigations 

 have been made by introducing the measured amount of sulphur 

 dioxide into a closed space containing the plant to be tested. In the 

 Commission's experiments the atmosphere was moved through the 

 test chamber and was maintained at the proper concentration. It 

 appears that there is a marked absorption of the sulphur dioxide both 

 by the plant tissues and by the soiP and it is probable that the earlier 

 investigators had in their containers far lower atmospheric concentra- 

 tions of sulphur dioxide than they supposed to be present. Both this 

 absorption of sulphur dioxide by the plant and the results of widely 

 separated intermittent fumigations suggest that the injury is really 

 determined by the balance of two rates ; the rate at which sulphur 

 dioxide is absorbed by the plant, and the rate at which it is removed 

 by physiological processes, given off again to the atmosphere or other- 

 wise disposed of. Doubtless both of these rates are affected by many 

 factors, some of which are undoubtedly internal to the plant. It is 

 easy to understand the discrepancies in the work of previous inves- 

 tigators and to see why the determination of a definite concentration 

 limit for toxicity has proved impossible. 



The Commission's experts were unable to establish the reality of the 

 so-called "invisible injury," a hypothetical decrease of yield supposed 

 to be due to a toxic effect too slight to produce visible lesions. Ap- 

 parently no decrease of yield occurs without easily visible injury exist- 

 ing or having existed. Indeed in several cases slight visible injury was 

 observed but was followed by no decrease of yield. 



2 This was confirmed by collateral laboratory investigations by G. C. Bartells, 

 Jr., reported on pp. 308-323. 



