Figure 6.— Lab study. Control, ponderosa pine, 

 longitudinal section, X300. Endodermis (EN), 

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

 xylem (X) are fully turgid. Hypertrophy and hy- 

 perplasia of parenchymatous tissue have not 

 occurred here, but do in needles affected by 

 phytotoxic gases. Current-year foliage. 



Figure 7.— Lab study. Control, ponderosa pine, 

 transverse section, X300. Note position of 

 epidermis (ED), hypodermis (HY), mesophyll 

 parenchyma (MP), and resin canal (RC). 

 Presence of numerous dispersed plastids in 

 turgid mesophyll cells and nonhypertrophied 

 epithelial cells (EP) in the resin canal are in- 

 dicative of a healthy needle. Current-year 

 foliage. 



Figure 8.— Lab study. Control, ponderosa pine, 

 transverse section, X300. Note position of 

 phloem (P), vascular parenchyma (VP), xylem (X), 

 endodermis (EN), and mesophyll (MP). Endoder- 

 mis is turgid and phloem and vascular paren- 

 chyma are not deeply stained nor hypertrophied. 

 Mesophyll is turgid and packed with plastids. 

 Current-year foliage. 



Figure 9.— Lab study. Salt injury, ponderosa 

 pine, longitudinal section, X300. Mesophyll 

 parenchyma (MP) collapsed, but the endodermis 

 (EN) remained turgid even when in contact with 

 necrotic mesophyll cells. Note the junction be- 

 tween necrotic mesophyll and healthy endoder- 

 mis (arrow). Current-year foliage. 



Figure 10.— Lab study. Salt injury, ponderosa 

 pine, transverse section, X300. This section is 

 from the necrotic portion of the transition zone. 

 Parenchymatous tissues including mesophyll 

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

 collapsed. Even in this condition of extreme in- 

 jury, the endodermis (EN) is, for the most part, 

 somewhat turgid. Current-year foliage. 



Figure 11.— Lab study. Drought, ponderosa 

 pine, longitudinal section, X300. Mesophyll (MP) 

 collapsed but endodermis (EN) in contact with 

 necrotic mesophyll remains turgid. Current-year 

 foliage. 



