Plant Qrotvth'Suhstances 



coleoptile test, limiting doses have been expressed as milli- 

 grams per plant (or what is in practice the same thing, a 

 prescribed degree of bending of so many plants per milli- 

 gram or gram of growth-substance), while the limiting values 

 for root-inhibition have been recorded as milligrams per litre 

 of solution, with either the number of plants or the volume 

 of solution left unspecified. 



Grace (1937, C) has made an attempt to solve the paradox 

 of growth-inhibition caused by the synthetic growth-sub- 

 stances. He concluded that inhibition of growth was usually 

 due to overdosage. His colleagues Newton and Jack found 

 that as little as i part of a-naphthyl-acetic acid in 200,000,000 

 parts of water depressed the growth of wheat roots in length, 

 though not in weight. Grace's dust treatment of seed (see 

 page 46) apparently gets round the paradox by slowly releas- 

 ing the active agent in amounts so small as to be stimulatory. 



An excellent review of the subject has been presented by 

 Borgstrom (1939) who has performed numerous experiments 

 in an attempt to resolve the paradox. He used genetically 

 uniform material [Allium) to determine the limits of con- 

 centration of various substances for the retardation or promo- 

 tion of growth. Inhibition at relatively high concentrations 

 was succeeded by a slight promotion at very dilute ones.^ 

 Like most other workers in this field, Borgstrom worked with 

 roots attached to the plant, but he has carefully reviewed the 

 possible interactions between applied synthetics and the 

 plant's own hormones. 



Borgstrom has also reviewed phytological eflFects of the 

 vitamins ascorbic acid (C) and thiamin (Bj). The effects of 

 these substances on roots is not restricted to a narrow range of 

 concentrations. He suggests, therefore, that they may have a 

 practical application, as they are "more convenient and far 

 safer to use as actually growth-promoting chemicals for roots". 



'On page 79 the author has pointed out that there is no comparability 

 between results of tests on roots and shoots, just because there is no common 

 unit. Borgstrom has made a slip in thinking that the author brought this 

 contrast forward as an explanation of the paradox discussed in this chapter. 

 The author had no other intention than to make a plea to future workers to 

 use consistent, or at least comparable, modes of stating their results. 



80 



