F. W. WENT 69 



It should be stressed that all these considerations, showing how essential 

 the idea of chemical messengers was for the interpretation of phenomena 

 observed in plants, were published 10-25 years before the hormone 

 concept was introduced in animal physiology. If the other botanists 

 of that time had been more constructive in their thinking and less of 

 the flaw-picking variety, plant hormones might have been a reality 

 long before the more tedious proofs for the existence of hormones in 

 animals had been produced. However, there should be no crying over 

 spilt milk; we should try to learn by the achievements and the faults of 

 our predecessors. 



After this first brilliant period in which from so many different angles 

 evidence for the existence of correlation carriers in plants was adduced, 

 and after the first reaction to this in the form of negations, a period of 

 consolidation started. Many investigators (Haberlandt, Ricca, Errera), 

 among whom I should like to single out two, J. Loeb and H. Fitting, 

 accumulated facts which demonstrated the action of correlation carriers. 

 But all remained isolated instances which only during the last 20 years 

 could be integrated. 



Loeb (14), the zoologist, became fascinated with the regeneration of 

 buds on severed leaves of Bryophylhtm and always had his laboratory 

 window sills full of those plants. For years he experimented with them, 

 came to the conclusion that this regeneration, the geotropic response of 

 the stems, and the outgrowth of axillary buds were all regulated by hor- 

 mones, and in several papers he presented evidence that these responses 

 all might be due to a single agent. It was impossible, however, to get 

 direct evidence concerning the nature of this agent, and thus Loeb (15) 

 rejected his earlier idea that a plant hormone was involved, and he 

 assumed that all his observations were an expression of a mass action 

 of some food constituent. 



In the work of Hans Fitting (10) we see the opposite development of 

 ideas. In an extensive investigation of the transmission of tropistic stimuli 

 in the Avena coleoptile he rejected any suggestion that this might be 

 accomplished by chemical or even physical means. He believed that his 

 experiments showed that light induced a polarization in cells, which 

 could be transmitted from cell to cell, even when by complicated 

 incisions there was no continuous linear connection between the stimu- 

 lated and reacting cells. This was different from the results of Darwin, 



