GROWTH AND GROWTH HORMONES IN PLANTS 403 



tion may gradually become insensitive to carbon monoxide while the water 

 uptake remains strongly inhibited by this poisonous gas. The appearance is 

 rather of a specific fraction of the respiratory process being in control of 

 water uptake or cell enlargement. The possibility that this respiratory frag- 

 ment is specifically using energy to draw water molecules into the cell has 

 been examined, but the evidence is against it; apparently the energy is used 

 instead to modify the properties of the cell wall (or perhaps of the cytoplasmic 

 membrane), and the water then enters as a result of the lessened wall resist- 

 ance. This too may explain why animal cells are not caused to grow by 

 auxin, although some evidence has been offered that carbohydrate metabolism 

 and glycogen deposition may be modified in liver tissues by auxin treatment. 

 The recent discovery of the presence of excessive quantities of 5-hydroxy- 

 indoleacetic acid in certain human pathological conditions may prove to be 

 of interest in this connection, for the substance has definite, though low, 

 auxin activity. 



It is a far cry from the phototropism of etiolated Avena coleoptiles to these 

 last-mentioned considerations, and the path traveled has been a long and 

 sometimes a winding one. It certainly could not have been foreseen, or even 

 wildly guessed at, in 1906, although Julius Sachs had even earlier had some 

 glimpse of it in his prophetic idea of '^organ-forming substances." Perhaps 

 what would have surprised the botanists of 1906 the most would be the 

 large practical applications of the auxins and of our knowledge of their 

 physiological actions, in horticulture and plant industry. The auxins have 

 provided also a powerful experimental tool for the attack on many long- 

 standing botanical problems, especially the nature of growth itself. And they 

 have greatly illuminated our general understanding of the life and behavior 

 of plants. 



LITERATURE CITED 



Books 



AuDUS, L. J. 1953. Plant growth substances. Leonard Hill. London. 



Leopold, A. C. 1955. Auxins and plant growth. Univ. California Press. Berkeley, 

 Calif. 



Skoog, F. (Ed.). 1951. Plant growth substances. (Symposium.) Univ. Wisconsin 

 Press. Madison. 



SoDiNG, H. 1952. Die Wuchsstofflehre. Georg Thieme. Stuttgart. 



Thimann, K. V. 1956. L'origine et les fonctions des auxines. Centre de Documenta- 

 tion Universitaire. Paris. 



Went, F. W., and K. V. Thimann. 1937. Phytohormones. Macmillan. New York. 



Recent Reviews 



Aberg, B. 1957. Auxin relations in roots. Ann. Rev. Plant Physiol. 8:153-180. 

 Addicott, F. T., and R. S. Lynch. 1955. Physiology of abscission. Ann. Rev. Plant 

 Physiol. 6:211-238. 



