4 Fundamentals of Auxin Action 



of formation to a site of action. In contrast to some compounds which 

 act as substrates for general physiological processes, hormones influ- 

 ence "a specific physiological process," and in each case minute amounts 

 produce a large physiological effect. It is evident that, by this definition, 

 the word hormone cannot apply to chemicals produced exclusively in 

 a laboratory, nor to such indigenous compounds as sugars, amino acids 

 or other substrates for growth. Although these latter types of com- 

 pounds may be transported, none of them are specific in their control 

 of physiological process, as far as is known. The status of vitamins in 

 this nomenclature is not clear. They are considered to be phytohor- 

 mones by some (Bonner and Bonner, 1948) in cases where they are 

 transported in the plant. 



A phytohormone, or plant hormone, by simple projection of the 

 preceding definition, is a hormone produced specifically in plants. 

 Within this category lie the growth hormone, the still-theoretical 

 flowering hormone, and possibly some vitamins. A few other phytohor- 

 mones have been proposed from time to time, but these first two are 

 the only ones generally recognized at present. Thimann (1948) has 

 defined a pliytohormone as "an organic substance produced naturally 

 in higher plants, controlling growth or other physiological function at 

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

 amounts." 



The groxvth hormone is the phytohormone which is involved in 

 growth. It is normally essential to growth by cellular enlargement, 

 both in length and in width, and is essential to growth of organs — 

 buds, stems, roots, fruits, and so on. 



The term auxin was originally suggested to refer to substances 

 which were capable of promoting growth in the manner of the 

 growth hormone (Kogl & Haagen-Smit, 1931). The search for the 

 actual growth hormone in plants led to the isolation of the first pure 

 active compound, not as the indigenous active hormone, but instead 

 as an ingredient of such miscellaneous biological materials as urine, 

 corn oil, and malt extract. The first such compound was named auxin 

 a, the second auxin b. In keeping with the original intent of the term, 

 auxin will be used to refer to "an organic substance which promotes 

 growth (i.e. irreversible increase in volume) along the longitudinal 

 axis when applied in low concentrations to shoots of plants freed as 

 far as practicable from their own inherent growth-promoting sub- 

 stances" (Thimann, 1948). This growth promotion may be conve- 

 niently measured by any of the standard auxin assay methods described 

 in chapter II. The growth hormone itself is an auxin, and so are any 

 other chemical compounds which can bring about the same growth 

 effect. 



