4 PLANT GROWTH SUBSTANCES 



minute amounts" (50). This definition includes those auxins which are 

 of natural occurrence, certain of the vitamins, and other hormones such 

 as those stimulating wound growth and the postulated hormones of 

 flowering. It brings in certain restrictions regarding the place of forma- 

 tion relative to the place of action not present in the original hormone 

 concept of Fitting (13, 14). Since industry has recognized the sales value 

 of the term hormone, many synthetic preparations appear under this 

 name. It has, therefore, been suggested that the requirement for natural 

 occurrence be omitted, thereby including the synthetic substances found 

 to have growth effects on plants. This meeting of many plant-hormone 

 workers might effect some agreement as to the proper use of the terms. 



The first efforts to determine the chemical nature of plant growth 

 stimulators were made by Fitting (13, 14) forty years ago. While visiting 

 the Dutch East Indies he made a study of the effects of poUination on the 

 orchid flower and noticed that pollen initiated swelUng of the ovaries 

 and other phenomena of post-flo ration. These effects could be reproduced 

 by the application of dead pollen as well as by water extracts of poflen. 

 A test method was devised whereby pieces of cotton wool soaked in the 

 active extracts were brought in contact with the stigma, after which 

 the growth effects could be observed. Fitting made several attempts 

 to fractionate the pollen extract and found that the hormone was 

 soluble in alcohol but insoluble in petroleum ether and ether. He even 

 looked for sources other than orchid pollen and found that saliva caused 

 similar responses. 



Although Fitting promised the continuation of his work, no further 

 data were available until Laibach (34) showed that the active pollen 

 substance was probably identical with one of the auxins. If Fitting had 

 continued his studies the history of the plant hormones might have 

 been quite different. After discovering the activity of saliva other excre- 

 tions such as urine might have been tried, and using Fitting's test 

 method, the auxins might have been isolated. Their parthenocarpic 

 effect might then have been discovered directly. Other actions such as 

 cell elongation in Avena, root formation, and the like, would have been 

 recognized as additional properties of the pollen hormone. There were 

 several reasons why history did not take this course. The microchemical 

 techniques were in their infancy, although Pregl at just about the same 

 time developed his organic microanalytical methods. The available physi- 

 ological tests would not have been adequate for the accuracy and the 



