244 



Journal of Agricultural Research 



Vol. XXII, No. s 



to 1 8°, 19° to 22°, 24° to 26°, 27° to 29°, 30° to 31°, 35° C. The moisture 

 content of the soil was held at two- thirds the water-holding capacity (22 

 per cent of dry weight). The air temperature of the greenhouse was 

 kept at about 15° with a rise to 20° during the middle of the day. These 

 conditions as to soil moisture and air temperature were such as had 

 proved favorable for both host and parasite development in the earlier 

 trials. The first seedlings to appear above ground were those at 27° 

 to 29°, those at 24° to 26° came up shortly afterward, then those at 

 19° to 22°. Good growth took place at these three temperatures, but 

 germination was very slow at lower temperatures. At the highest tem- 

 perature, 35°, a few seeds germinated, but growth was very slight. The 

 plants were all removed and the roots washed out on January 7, 

 1920, 29 days after the seed was sown. The data given in Table IV 

 summarize the condition of tlie plants at this date. It will be seen that 

 at this early stage in the development of the plants there was a tendency 

 for best root development at about 21° or below, while the best develop- 

 ment of tops took place at this point or above. 



Table IV. — Development of onion seedlings in sterilized greenhotise soil held at 22 per 

 cent oj the dry weight or two-thirds the moisture-holding capacity, and at different soil 

 temperatures. Data on January 7, IQ20, 2Q days after sowing 



Soil temperature. 



"C. 

 10 to 14 . . . 

 16.5 to 18. . 

 19 to 22 . . . 

 24 to 26. . . 

 27 to 29 . . . 

 30 to 31. . . 



Average 



total dry 



weight per 



plan I. . 



Gm. 



0.0027 

 .0031 

 .0044 

 •0033 

 .0030 

 .002 1 



» The reduced stand at 27° to 29° and 30° to 31° C. was due to damping-off fungi. 



Experiment II. — ^The experiment was repeated, with some modifica- 

 tions, starting April lo, 1920. The Red Globe and Yellow Bermuda 

 varieties were used. Two pots of each variety were kept at each of the 

 following soil temperatures: 14°, 20°, 25°, 28°, 30° C. The air temper- 

 ature ran sHghtly higher (20° to 30°) during the middle of the day and 

 dropped to about 15° for the most of the night. Both the rate and the 

 percentage of germination were noted, and the data are recorded in 

 Table V. In both varieties the most rapid germination took place at 25°, 

 although the rate was only slightly less at 20°, 28°, and 30°. At 14° the 

 seedlings were distinctly slower in starting ofif. The plants from one pot 

 of each series were removed on the thirteenth day. The dry weights of 

 the tops and roots as given in Table V were so small at this age that com- 

 parison on this basis does not have any great value. The tendency for 

 rapid development of tops as compared with roots at 20° or above is, 

 however, shown very strikingly in Plate 25. The plants in the remaining 



