Figure 2.— Lab study. Control ponderosa pine 

 seedling after treatment with clean air in 

 fumigation chamber. Current-year foliage re- 

 mained green; no necrosis or chlorosis was 

 observed. 



Figure 3.— Lab study. Ponderosa pine seedling 

 after treatment with 5 ppm S0 2 for 6 hours. All 

 current foliage developed necrosis. 



Figure 4.— Lab study. Control Douglas-fir seed- 

 ling. Similar to ponderosa pine, current-year 

 foliage remained green and appeared healthy. 



Figure 5.— Lab study. Douglas-fir foliage after 

 fumigation with 5 ppm S0 2 for 6 hours. Note the 

 well-defined tip necrosis. Needle segments for 

 histological study were taken from the transi- 

 tion zone, which included green, chlorotic, and 

 necrotic tissue. Serial longitudinal and trans- 

 verse thin sections 4.72 x 10" 4 in (12 microns) 

 thick then were made through the entire transi- 

 tion zone. Histological interpretations were 

 made from these sections. 



