Development of Knowledge of Auxins 5 



The term growth regulator refers to organic compounds other 

 than nutrients, small amounts of which are capable of modifying 

 growth. Included in this category are substances which either stimu- 

 late, inhibit, or otherwise alter growth. It is presumed that growth 

 regulators act upon growth by altering the net effect of the growth 

 hormone, though this is speculative in many cases. Included as growth 

 regulators are such substances as auxins, anti-auxins, epinastic agents 

 and other types of materials as discussed in chapter VII. 



In the early literature, the term heteroauxin was used to refer to 

 indoleacetic acid (Kogl et al, 1934). Since the time when this term 

 was suggested, a great quantity of evidence has accumulated indicat- 

 ing that indoleacetic acid is indeed the commonest growth hormone, 

 and hence to classify it as an "outsider" or heteroauxin seems entirely 

 misleading. In accord with the suggestion by Thimann (1948), we 

 will not use the term in the present discussion. 



Unfortunately there has been a considerable usage of the term 

 hormone in referring to synthetic auxins or other growth regulators. 

 This has been done largely to strike an appeal to potential buyers of 

 commercial products containing auxins. Such a misuse of terms is 

 definitely confusing and has no place in scientific writing. The term 

 hormone will be used in the present discussion to refer only to sub- 

 stances identical with those known to be indigenous to the plant, and 

 known to act as hormones in the plant. 



TROPISMS AND THE EARLY WORKERS 



When Charles Darwin turned his brilliant mind to the study of 

 plant movement and tropisms, the first glimmering of the existence 

 of the growth hormone was revealed. Darwin's simple and logical 

 experiments, using canary-grass seedlings, a light source and a razor 

 blade, told him that the tip of the shoot is involved in the overall 

 tropic response. Removal of the tip was followed by loss of sensitivity 

 to light in the coleoptile below. He concluded (1881) that "when seed- 

 lings are freely exposed to a lateral light, some influence is transmitted 

 from the upper to the lower part, causing the latter to bend." 



Darwin's work aroused much interest and discussion, and led 

 eventually to the work of Boysen- Jensen (1913) in Germany, who 

 found that although severing the oat coleoptile tip removed photo- 

 tropic sensitivity, simply replacing the tip restored the sensitivity 

 again. He concluded that "the transmission of the irritation is of a 

 material nature produced by concentration changes in the coleoptile 

 tip." 



Careful repetition of Boysen-Jensen's work in turn by Paal (1919), 



