2 BULLETIN 312, U. S. DEPARTMENT OF AGEICULTURE. 



The first problem, t4ierefore, seemed to be to increase the leaf 

 area and thus increase the yield of oil per acre. Accordingly, in 1910 

 a small plat of ground was set with plants grown from a mixed col- 

 lection of seeds secured from wild plants in Volusia County. Of the 

 first year's growth from this plat, 196 pounds of herb was distilled and 

 yielded 0.17 per cent of oil. In the second year the yield of oil from 

 plants grown on the same plat was 0.24 per cent. The content of 

 total phenols in the oils was found to be 64 per cent. 



A large number of variations in the plants were noted in 1910, and 

 from these about sixty individual plants showing the various forms 

 were selected for seed purposes. The seed from these plants was sown 

 in 1911, but the resulting plants were not true to type. It was noted, 

 however, that the variations fell into two general classes — dark and 

 light plants — and that to a large extent these variations came true 

 to type. The dark plants had much darker leaves, more serrate, 

 and with a pronounced red color in the stems and branches. The 

 light plants had larger leaves, lighter green in color, and with little 

 or no red color in the stems and branches. There was also a greater 

 tendency to the production of leaves in the light than in the dark 

 type. From 34 tests which were made to show the relative yield of 

 oil of these two types of plants the following average yields of oil 

 were obtained: From the dark type, 0.40 per cent of oil, which 

 contained 66 per cent of phenols; from the hght type, 0.42 per cent 

 of oil, with 69 per cent of phenols. The specific gravity, as well as 

 the percentages of oil and of phenols in the oil, was found to be higher 

 in the plants of the light type. Future seed selection was therefore 

 carried on with this type. 



In 1912 a further comparison was made between the light and dark 

 forms and the results obtained were still more marked, but owing 

 to weather conditions all samples showed a low yield of oil. The aver- 

 age of 36 tests was as follows: Dark type, 0.31 per cent of oil, which 

 contained 65 per cent of phenols; light type, 0.40 per cent of oil, 

 which contained 68 per cent of phenols. By continuing the selection 

 of the light type of plants the yield of oil has been gradually increased, 

 and in 1914 the herb from a plat of two-thirds of an acre yielded 0.44 

 per cent of oil, which contained 72 per cent of phenols, while in 1915 

 the average was 0.42 per cent of oil, containing 74 per cent of phenols. 



During these years selection was also made to increase the size of 

 the plants in order that the tonnage of herb per acre might be in- 

 creased. This was also successful and a considerably increased yield 

 was noted year by year. It is believed that at the present time the 

 selection has been carried far enough to warrant the use of the im- 

 proved form for the commercial production of thymol in the United 

 States.^ 



1 The department has no supply ot selected seed or plants of horsemint available for distribution. 



