II DEFINITIONS 753 



in higher plants, controlHng growth at a site remote from its place of production, 

 and active in minute amounts. This definition, proposed to minimize confiasion 

 (Thimann et al., 1952) has been widely adopted and is parallel to similar defini- 

 tions used for animal hormones. Similar growth hormones have not been demon- 

 strated in fungi. 



An auxin is an organic substance which promotes growth along the longitudinal 

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

 from their own inherent growth-promoting substance. Auxins may, and generally 

 do, have other properties, but this one is critical. Most auxins have a characteristic 

 chemical structure (see section IV, p. 760) and are aromatic acids or their deriva- 

 tives, but there are one or two aliphatic avixins. Fvirthermore the gibberellins, 

 which are chemically very different, have many of the typical properties of auxins. 



The definition, whose significance will be clearer after the discussion of auxins 

 and their bioassay, excludes substances acting only to activate auxins already 

 present, as for instance ordinary acids, causing marked growth effects in some green 

 tissues, and also the ions of potassium and cobalt. It also excludes nutrients such 

 as sucrose, which have to be supplied in relatively high concentrations. 



Where action on roots is concerned, the effects of many synthetic auxins can be 

 complex. In general auxins inhibit root elongation. Some auxin-like compounds 

 antagonize the effects of true auxins. The resulting problems of definition, especially 

 of the terms "auxin antagonist" and "root auxin", are taken vip in section VII b 

 (p. 801). 



Kinins are substances of recent discovery which promote growth when acting 

 in conjunction with auxin. The resulting growth is not typically along the longi- 

 tudinal axis and the kinins are believed to act primarily by promoting cell division. 

 Chemically they differ from the auxins, and are related to the purines (see section 

 V d, p, 773). Both auxins and gibberellins also promote cell division in specific 

 instances. 



A meristem is a mass of undifferentiated tissue whose cells are actively dividing, 

 or potentially capable of active division. 



III. THE MEASUREMENT OF GROWTH 



(fl) General 



In the 19th century the distribution of growth along a plant or a plant organ 

 was measured by making a series of small ink marks at equal distances apart (say 

 I mm) and measuring their distances apart at the end of the growth period. In 

 this way Sachs, in the i88o's, demonstrated that the region of elongation in the 

 seedling root is not at the tip but begins 1-2 mm behind the tip and extends for 

 several mm backwards. The extreme tip does not elongate appreciably, nor does 

 the part 2 cm behind the tip. The intervening zone, especially its more apical part, 

 may elongate several hundred per cent in 24 h. The method is still in use for locali- 

 zation of growth zones. Some recent applications were by Went (1928) and Curry 

 et al. (1956) {or Avena coleoptiles. Van Overbeek and Went (1937) for isolated 

 pea stem sections, and Gunckel et al. (1949) for leafy shoots oi Ginkgo. 



Another method much in favor was the attachment of the tip of a growing 



Literature p. 8i6 



