Robert A. Steinberg 379 



leaf lamina growth. Figures 2 and 3 illustrate frenching of tobacco in 

 the field and the isoleucine effect in aseptic culture, respectively. 



Hydroxyproline at 5 p.p.m. quickly killed. Various types of chloroses, 

 necroses, and abnormalities in leaf form were characteristic of other 



Figure 2. Field of tobacco in Lancaster County, Pennsylvania, showing 

 extreme frenching. — Courtesy of Dr. O. E. Street. 



amino acids. The conclusion was drawn that production of symptoms 

 in the plant was probably due to abnormal protein metabolism and the 

 excessive accumulation of metabolites. A later paper (49) confirmed 

 and extended these results to possible aliphatic acid metabolites. Only 

 the natural isoleucine was found to be effective, and not the unnatural 

 optical isomer (Figure 4). 



A still later paper (50) dealing with field plants of tobacco showing 

 symptoms of frenching and mineral deficiencies affords additional 

 evidence for this interpretation. Symptoms of frenching and of calcium, 

 magnesium, potassium, phosphorus, and boron deficiency were ac- 

 companied in each case by marked increases in free amino acid content 

 of the leaf lamina. These increases ranged up to about 600 per cent: 



