The History and Nature of Plant Growth Hormones 



A. J. HAAGEN-SMIT 



A LOOK at the index of the Biological Abstracts provides some 

 indication of the general interest in the field of plant growth 

 substances. In 1930 only fifty papers appeared, but now this number has 

 increased to a few hundred a year. 



With the growing number of papers on this subject, there has arisen 

 a confusion in the use of the terms growth substance, growth hormone, 

 regulator, phytohormone, formative substance, and auxin. To avoid 

 further confusion I shall adopt Thimann's (50) recent suggestion on the 

 nomenclature of the auxins and phytohormones, which is as follows: 

 "An auxin is an organic substance which promotes growth (i.e. irre- 

 versible increase in volume) along the longitudinal axis, when applied 

 in low concentrations to shoots of plants freed as far as practical from 

 their own inherent growth promoting substances. Auxins may, and 

 generally do have other properties, but this one is critical." 



This definition excludes sugars which promote longitudinal growth 

 when the term "low concentration" is interpreted as meaning below 

 0.00 1 molar. It also excludes nutrient salts and is intended to exclude 

 substances such as mafic and other organic acids which promote growth 

 of the Avena coleoptile in a 0.00 1 molar concentration and less, but 

 only in the presence of auxin. In this concept, van Overbeek's (46) 

 criterion that an auxin should be active in the Avena test is replaced by 

 a more general statement on the longitudinal growth of shoots. This 

 change made it possible to include also substances active in other tests 

 such as the pea test. 



A phytohormone has been defined as: "An organic substance produced 

 naturaUy in higher plants, controUing growth or other physiological 

 functions at a site remote from its place of production, and active in 



