8 PLANT GROWTH SUBSTANCES 



of corn, tomato, radish, and other plants. In testing for the inhibiting 

 action on Avena coleoptiles it is necessary to supply the plant with 

 sufficient growth hormone so that depression of the activity can be 

 measured. Larsen (38) measured in this way the inhibitory effects of the 

 seed-germination inhibitors, anemonin andparasorbic acid. Effects were 

 noticed, respectively, in concentrations of o.i gram per liter and of 

 i.o gram per liter. The biological activity of a given amount of indole- 

 acetic acid is neutralized in the Avena test by 2x10^ to 2x10^ times the 

 amount of parasorbic acid. The number of known, naturally occurring, 

 inhibiting substances is rapidly increasing, and future determination 

 of the auxin content of plant and animal extracts must take these facts 

 into account. It seems necessary, therefore, that in these determinations 

 activity curves be compared with those of known growth substances. 

 If a significant difference occurs it will then be necessary to purify the 

 extracts before measuring their activity. Recovery experiments, whereby 

 a certain amount of synthetic growth hormone is added to the extract 

 to be analyzed, will also be of value. Such a procedure is to be recom- 

 mended in any auxin determination where extractions have been carried 

 out. The large amounts of solvents used to extract the exceedingly small 

 quantities of hormone may easily introduce considerable uncertainty 

 about the result. A routine treatment of the ether with peroxide- 

 destroying agents is not a guarantee for the reliability of the results. 

 A good example of the effect of inhibitors on the curvature in the 

 Avena test is found in a study made by Larsen (38). The addition of a 

 neutral extract of tomatoes depresses the curve of both auxins consider- 

 ably. The acid growth substance can be easily separated from such a 

 neutral inhibitor. However, this is not possible with the neutral auxins. 

 To compUcate matters further, inhibitors have been found which are 

 acidic in nature. In such cases a comparison of growth curve of the pure 

 auxin and that of the crude extract must be made in order to interpret 

 the results correctly. 



The Avena test made possible important discoveries in the field of 

 geotropism and phototropism. It was shown that these phenomena 

 depended on the presence of the growth hormone and its lateral dis- 

 placement. As a direct result of the availability of a testing procedure 

 we may also list attempts to isolate the growth substances. The first 

 was made by Nielsen (40,41) who found that two pathogenic fungi 

 produced in the nutrient medium substances which were strongly active 



