FIFTY YEARS OF PLANT PHYSIOLOGY IN THE U.S.A. 619 



that the universality of investigations of O2 and CO2 exchange, to the exclusion 

 of most other gases, is due to the limitations of the Warburg technique. It 

 is to be hoped that the development of gas chromatography will bring as 

 healthy a re-evaluation of the general problem of gas exchange as column 

 and paper chromatography has achieved in the general field of metabolism. 



Special mention should be made of the advances in the field of electron 

 diffraction, which produced the electron microscope. This instrument has 

 opened completely new possibilities in correlating structure and function, 

 because it brings visibility down almost to the molecular level. The problem of 

 cell growth has been revived by electron microscope studies of the cell wall, 

 and it seems certain that in the next few years the function of cytoplasm, 

 plastids, and nucleus will become much better understood with the help of 

 the electron microscope. It is hard to believe now that in the 1920s it was 

 generally accepted that no further improvements in microscopic observation 

 could be expected. This shows how one simple conclusion of Dirac, that 

 particle streams could have wave properties, has opened completely new 

 horizons in biology. 



As a plant physiologist one must regret that other branches of physics have 

 not kept pace with the development of nuclear physics. It is certain that any 

 new light which could be shed on the properties of water, on ion and electron 

 transfer in solid and liquid phases, or on diffusion phenomena would be 

 eagerly welcomed by physiologists and very likely would lead to important 

 theoretical and practical advances in water-relation problems and cellular 

 metabolism. A historical analysis like the foregoing practically proves this. 



The relationships between physical chemistry and chemistry, on the one 

 hand, and plant physiology, on the other hand, have traditionally been very 

 close, and if possible have become still closer in the 20th century. Here again 

 excesses have occurred, and applications have sometimes assumed fad-like 

 proportions. In the 1920s pH measurements became a panacea for solving 

 physiological problems, and at present many persons seem to labor under 

 the misapprehension that plant physiology is identical with biochemistry. 

 I would like to emphasize my personal conviction that physiology is the 

 analysis and synthesis of life phenomena in terms of individual reactions or 

 processes. Once such a reaction or process is isolated from the behavior of the 

 organism as a whole, the biochemist or biophysicist can further identify the 

 individual process. Without a proper physiological analysis, however, bio- 

 chemical studies are meaningless. By diligent grinding of different plants a 

 whole number of auxin-destroying and auxin-synthesizing enzymes have been 

 discovered. Since the necessary physiological studies have not preceded — or 

 even followed — the biochemical work, the significance of these enzymes in 

 the life of the plant is a complete mystery, and thus this biochemical evidence 

 has no obvious relation to our understanding of the functioning of the plant. 



Among the most important advances in chemistry, which have had imme- 



