INTRODUCTION AND HISTORICAL SKETCH 3 



substances, i.e., Wuchsstoffe^, have been referred to by various 

 workers as growth hormones, growth regulators, growth enzymes, 

 phytohormones, and auxins. They inehide compounds that 

 promote the growth of the Avena coleoptile and the hypocotyls, 

 stems, and leaves of various dicotyledonous plants, but they 

 apparently retard the growth of roots. They are known to be 

 produced by Avena, Zea, Rhizopus, Aspergillus, various bacteria, 

 and numerous other organisms. They are ether-soluble, sensi- 

 tive to peroxide, and have an acid character. 



HISTORICAL SKETCH 



Botanists first became acquainted with growth substances 

 through studies on tropisms, i.e., those growth curvatures that 

 take place in response to unilateral stimulation of an organ by 

 light or its displacement from the usual position of equilibrium 

 with respect to the force of gravity, etc, A short historical 

 survey of the earlier contributions to our knowledge of the growth 

 phenomena concerned in photo- and geotropism is presented here 

 as a background for the information that will follow. 



Darwin. — In a book entitled "The Power of Movement in 

 Plants," Darwin (1881) recorded extremely valuable experi- 

 ments and reflections upon the movements of plants in response 

 to light. Among other things, he demonstrated a localization 

 of the phototropic stimulus in certain plants. The main object on 

 which he experimented was the coleoptile of Phalaris canariensis. 

 When this organ was unilaterally illuminated, a strong positive 

 phototropic curvature resulted. If the tip of the coleoptile 

 was darkened by a tinfoil cap or a darkened glass cap, and only 

 the lower part was unilaterally illuminated, curvature usually 

 did not result. However, if the procedure was reversed, i.e., if 

 the upper part of the coleoptile was unilaterally illuminated while 

 the lower part was darkened (by means of moist sand), a photo- 

 tropic curvature took place in the lower portion (Fig. 1). It was 

 shown also that a coleoptile does not react phototropically when 

 2.5 to 4 mm. of the tip is removed (Fig. 1). Darwin concluded 

 (1881, p. 474) "that when seedlings are freely exposed to a lateral 

 light, some influence is transmitted from the upper to the lower 

 part, causing the latter to bend." Localized sensitivity to light 

 and conduction of a stimulus were observed also in the coleoptile 



1 This refers to Wuchsstoffc A. The term Wuchsstoffe B refers to a 

 different class of substances, such as Bios (see Nielsen, etc.. Supplementary 

 Bibliography) . 



