Figure 18.— Lab study. Ethyl mercaptan, pon- 

 derosa pine, longitudinal section, X300. Symp- 

 toms are similar to those caused by sulfur diox- 

 ide. Note hypertrophied vascular parenchyma 

 (VP), intense reddish staining in vascular bun- 

 dle, and endodermis (EN) collapse in contact 

 with necrotic mesophyll (MP). Current-year 

 foliage. 



Figure 19.— Lab study. Hydrogen fluoride, 

 Douglas-fir, longitudinal section, X300. Note 

 collapse of endodermal cell (EN) in contact with 

 necrotic mesophyll (MP). Also, the vascular 

 bundle is deeply stained. Symptoms are similar 

 to those caused by sulfur dioxide, hydrogen 

 sulfide, and ethyl mercaptan in ponderosa pine 

 needles. Compare with figures 14-18. In this ex- 

 periment, the different symptoms induced by 

 various treatments in ponderosa pine needles 

 were identical to those induced in Douglas-fir 

 needles. Current-year foliage. 



Figure 20.— Field study. Control, ponderosa 

 pine, transverse section, X125. Note position of 

 mesophyll (MP), endodermis (EN), phloem (P), 

 xylem (X), and vascular parenchyma (VP). The 

 only difference between this needle and the 

 control needle of the lab study (fig. 8) is that the 

 phloem and xylem are better developed. All 

 needles shown for field study were 2 years old. 



Figure 21 .— Field study. Winter drying, ponder- 

 osa pine, longitudinal section, X25. Note posi- 

 tion of epidermis (ED), mesophyll (MP), resin 

 canal (RC), endodermis (EN), vascular paren- 

 chyma (VP), phloem (P), and xylem (X). The col- 

 lapsed mesophyll (arrow) identifies the necrotic 

 part of the needle. No intensive, reddish stain- 

 ing occurred in the vascular bundle nor did 

 transfusion parenchyma undergo hypertrophy 

 and hyperplasia. 



Figure 22.— Field study. Winter drying, ponder- 

 osa pine, longitudinal section, X125. Note col- 

 lapsed mesophyll (MP). Endodermis (EN) in con- 

 tact with collapsed mesophyll is turgid, healthy, 

 and vascular parenchyma (VP) is normal. This 

 section included some epithelial (EP) tissue 

 that also appears normal. 



Figure 23.— Field study. Sulfur dioxide, ponder- 

 osa pine, longitudinal section, X25. Deeply 

 stained vascular tissue (VT), collapsed meso- 

 phyll (MP), and necrotic endodermal cells (EN) 

 occurred. This is similar to specimens injured 

 by sulfur dioxide in the lab study. 



