1. Evidence is presented to show that when guayule plants (6months to 2 years of age) are grown in gravel culture, an influenceis exerted which is inhibitory to the growth of guayule seedlings.
2. The development of two types of semi-quantitative assay forthe detection of the inhibiting principle is described.
3. A method is described for the collection of large quantitiesof nutrient solution containing the inhibiting principle, and for thesubsequent concentration of this material to small volume. It ispointed out that the production of the inhibiting principle decreasessharply during the winter months.
4. Evidence is presented to show that the inhibiting principleis an organic substance. The critical evidence consists of the disappearanceof activity on ashing and of the extractability of theinhibitor by organic solvents.
5. It is estimated that a guayule plant (6 months to 2 years ofage), growing in gravel culture gives up to the nutrient medium, onthe average, a minimum of 200 gammas per day of organic matter.
6. Various chemical properties of the inhibitor are given. Itis shown to be relatively stable to heat, stable to drying, non-volatile,non-adsorbable by charcoal, and probably non-precipitable by lead.
7. An experiment is described in which a crude crystalline materialwas obtained which was very active.
8. Evidence is presented to show that the effect of the inhibitoris not due to an influence on the pH of the nutrient solution.
9. The occurance of growth stimulatory organic substances inthe guayule leachate is described. These stimulatory fractions wereactive at concentrations as low as 2 mg. per liter.
10. A number of pure synthetic organic acids were tested forinhibitory activity, of these only one, cis-9,10 dihydroxy stearicacid was found to be active. This compound caused marked inhibitionin concentrations as low as 3.5 mg. per liter.
11. The chemical fractionation of a soil on which guayule plantshad grown is described and it is shown to contain inhibitory organicsubstances having some chemical properties similar to those of thesubstances in the guayule leachates.
