Figure 12.— Lab study. Winter drying, ponder- 

 osa pine, longitudinal section, X300. Mesophyll 

 (MP) collapsed, but endodermis (EN) remained 

 turgid. Note similarity between injury by salt, 

 drought, and winter drying (figs. 9-12). Current- 

 year foliage. 



Figure 13.— Lab study. Winter drying, ponder- 

 osa pine, transverse section, X300. Mesophyll 

 (MP), vascular parenchyma (VP), and phloem (P) 

 collapsed while endodermis remains turgid. No 

 deep staining occurred in the vascular tissues. 

 Current-year foliage. 



Figure 1 4.— Lab study. Sulfur dioxide, ponder- 

 osa pine, longitudinal section, X300. Mesophyll 

 (MP) collapsed and endodermis (EN) in contact 

 with collapsed necrotic mesophyll also col- 

 lapsed. Note the intense reddish staining in the 

 vascular bundle (within the endodermis) exten- 

 ding basipetally into the region of noninjured 

 mesophyll. Note the hypertrophy (excessive cell 

 enlargement [HTD and hyperplasia (excessive 

 cell division [HP]) that occurred in the vascular 

 parenchyma, causing the vascular bundle to 

 swell. These three symptoms, endodermis col- 

 lapse, intense vascular stain, and swelling, are 

 characteristic of injury induced by phytotoxic 

 gas, but not by other causes. Compare with salt, 

 drought, and winter drying (figs. 9-13). Current- 

 year foliage. 



Figure 15.— Lab study. Sulfur dioxide, ponder- 

 osa pine, transverse section, X300. This section 

 is from the green end of the transition zone. 

 Vascular parenchyma (VP) hypertrophied and 

 the characteristic deep reddish stain pervaded 

 the vascular tissues. Mesophyll (MP) has not 

 collapsed. Phloem (P) has been destroyed. 

 Current-year foliage. 



Figure 16.— Lab study. Sulfur dioxide, ponder- 

 osa pine, transverse section, X300. Epithelial 

 cells (EP) hypertrophied and occluded the resin 

 canal (RC). Mesophyll (MP) is not noticeably af- 

 fected. Hypertrophy of epithelial tissue com- 

 monly occurs in needles injured by phytotoxic 

 gases, but also is found in needles injured by 

 winter drying and drought. It is not distinctive 

 for phytotoxic gases. Current-year foliage. 



Figure 17.— Lab study. Hydrogen sulfide, pon- 

 derosa pine, longitudinal section, X300. Col- 

 lapsed endodermis (EN), deep-red vascular 

 staining, and hypertrophy of vascular paren- 

 chyma (VP) are evident. These symptoms are 

 similar to those caused by S0 2 - Current-year 

 foliage. 



