70 PLANT GROWTH SUBSTANCES 



Rothert, and others, and it seemed to disprove the existence of a correla- 

 tion carrier, or of electrical transmission of a stimulus. This hopelessly 

 confused the issue, so that for years its solution was impossible. 



But a few years later when Fitting (i i) worked in Buitenzorg, Java, on 

 the flowering of orchids, he found that the swelling of the ovary and the 

 fading of the flowers after pollination was due to a water-soluble, heat- 

 stable substance, which occurred in the pollinia, and which he compared 

 with a hormone. It is unfortunate that this paper of Fitting, which so 

 clearly showed the existence of a hormone-like compound in the plant 

 and proved that it could be handled outside the plant body like any other 

 chemical, had so little influence on botanists, whereas his paper on the 

 transmission of the light stimulus, which we might designate as almost 

 contrary to the facts as we know them now, had such a profound 

 influence. 



It was only natural that the work of Fitting stimulated PfefFer, and 

 he assigned Boysen Jensen, who was just then visiting his laboratory, 

 to repeat Fitting's work. Whereas Fitting had made deep incisions in 

 grass coleoptiles in many different ways without preventing transmission 

 of the phototropic stimulus, Boysen Jensen went one step further. In 

 a few experiments he completely severed the tip of an Avena coleoptile 

 and then replaced it. This drastic treatment did not prevent the trans- 

 mission of the phototropic stimulus either. This experiment invalidated 

 Fitting's hypothesis of transmission of a polarity of the coleoptile cells 

 induced by light. 



Four years later Boysen Jensen (3,4) published a full account of his 

 experiments in Pfeffer's laboratory and of additional work in Copenhagen 

 and discussed their impHcations. Within the framework of current ideas, 

 especially of Pfeffer and his school, the experiments did not fit very well. 

 According to them transmission of a stimulus was a complicated process, 

 comparable to the transmission of stimuH in animals, with the tacit 

 implication that some electrical phenomenon was involved. This seemed 

 to be contradicted by Boysen Jensen's experiment, but in its discussion 

 it was pointed out that potential differences could arise by concentration 

 differences. 



Thus in explaining transmission of the phototropic stimulus Boysen 

 Jensen arrived at the mental picture of the transmission of a concentra- 

 tion gradient of substances, which could pass a cut and which could 

 produce the necessary electrical gradient in the base of the coleoptile. 



