Figure 24.— Field study. Sulfur dioxide, ponder- 

 osa pine, transverse section, X125. Deep red- 

 dish stain (arrow) and necrotic endodermis (EN) 

 in contact with necrotic mesophyll (MP) typify 

 injury by phytotoxic gases. 



Figure 25.— Field study. Hydrogen sulfide, 

 ponderosa pine, longitudinal section, X125. 

 Note necrotic endodermis (EN), intense stain in 

 vascular tissue (VT), and hypertrophy of vascu- 

 lar parenchyma (VP). 



Figure 26.— Field study. Hydrogen sulfide, pon- 

 derosa pine, transverse section, X125. This sec- 

 tion is from the greenish portion of the transi- 

 tion zone. Note destruction and heavy staining 

 in the vascular tissues (VT). Note also the hyper- 

 trophy and hyperplasia in the vascular paren- 

 chyma (VP). 



Figure 27.— Field study. Hydrogen fluoride, 

 Scotch pine, longitudinal section, X125. These 

 specimens were obtained from the Rhone Valley 

 in Switzerland. Characteristic of injury caused 

 by fluoride and other phytotoxic gases, endo- 

 dermal cells (EN) in contact with necrotic meso- 

 phyll (MP) are necrotic, the vascular cylinder 

 (VC) is deeply stained, and vascular parenchyma 

 (VP) has hypertrophied. Compare with figures 14 

 through 19 and 23 through 26. 



Figure 28.— Field study, hydrogen fluoride, 

 Scotch pine, transverse section, XI 25. This 

 specimen also is from the Rhone Valley in 

 Switzerland. The section was cut in the greenish 

 portion of the transition zone. Notice the deep 

 staining of vascular parenchyma (VP). 



