K. V. THIMANN 



and sugar, and the accumulation of solutes by roots, have been greatly 

 clarified. Even the elusive field of photosynthesis has seen notable 

 progress through the investigation of the effect of intermittent light, 

 and the recent study of the "dark" reaction with radioactive isotopes. 



In no field has more striking progress been made than in that 

 of the analysis of growth and its control by hormones. Darwin's work 

 on what was later to become a classical material for study — the oat 

 coleoptile — may be regarded as the first serious attempt to analyze 

 growth. The work was refined and advanced during the fifty years 

 that followed. In the first "shoot" of the dark-grown oat seedling, the 

 extreme tip — a region of about 0.5 mm. in length — was shown (by 

 the combined efforts of a number of workers) to produce a diffusible 

 substance which controls the responses to light and gravity of the whole 

 shoot. Later it became clear that the curvatures toward light, or 

 away from the earth, which constitute these responses, are special 

 cases of normal or symmetrical growth. The growth substance or 

 hormone, diffu.sing from the tip into the base of the coleoptile, is 

 asymmetrically distributed, with the result that one side grows faster 

 than the other, causing curvature. In the absence of external stimulus, 

 the hormone is symmetrically distributed, and it promotes and con- 

 trols the normal elongation of the organ. 



Discovery of the growth-promoting activity in sources far 

 richer and more varied than the tips of seedlings, and the development 

 of quantitative methods for assay by Went and others, soon led to the 

 isolation of several "auxins" active in the test methods. Later it was 

 recognized that a large class of synthetic substances have auxin activity 

 (see below), an activity which is measured by methods that are now 

 standardized. The method most frequently used involves decapitating 

 the oat coleoptile, thus depriving it of its main source of auxin, and 

 applying the test material to one side. The curvature which results 

 is then measured. Alternatively, the auxin may be applied in such a 

 way as to induce straight growth, which is then measured under the 

 microscope. Small sections cut from the coleoptile, pea, or other 

 seedlings can be immersed in solutions of test substances and their 

 growth measured. Many other procedures have been used, all 

 involving the use of auxin-deficient material as the test object. 



"Auxin a" (or auxentriolic acid) and "auxin b" (or auxenolonic 

 acid) were isolated by KogI, Haagen-Smit, and Erxleben from urine 



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