Wire Stem of Cabbage 27 



With this arrangement and good thermostats, it was possible to keep the 

 temperature of the water bath constant so that an ordinary thermometer 

 registered no variation for weeks at a time (figure 9, page 41). The 

 temperature in the uncontrolled tank, if adjusted two or three times a 

 day, would fluctuate within two or three degrees. Thermographs were 

 installed in each tank. These, though fairly accurate, required consider- 

 able adjusting, and were not relied upon for temperature readings but were 

 used to indicate changes in temperature only. The temperature of 

 both the water bath and the soil in the culture vessels, as well as that of 

 the air, was taken several times daily by means of thermometers. While 

 the temperature of the water remained constant, it is evident that in an 

 ordinary greenhouse the soil temperature nevertheless varies, depending 

 on the intensity of the sunlight and the fluctuations of the air temperature. 

 The greater the difference between the air temperature and the water 

 temperature, the greater are the fluctuations in the soil temperature and 

 the greater is the difference between the soil and the water temperature. 

 For example, when it was desired to maintain the soil at 32° C, it was 

 necessary to maintain the water at 35°, since the house was kept at approxi- 

 mately 10° to 15° C. at night and at about five to ten degrees warmer 

 during the day. However, the air temperature was extremely variable, and 

 bright sunlight would cause the soil in this tank to reach a temperature 

 somewhere between 33° and 34° C. As soon as the direct rays of the sun 

 were no longer on the culture vessels, the temperature would drop rapidly 

 to approximately 32°. The limits were, however, never higher than as indi- 

 cated in the tables, and usually were lower. The temperatures given in 

 the tables are the maxima and minima taken in the upper half-inch of 

 the soil, as this is the region most directly related to damping-off. Various 

 measures, such as maintaining the surface of the soil an inch or more 

 below the surface of the water, and planting the seedlings close to the 

 sides of the jar, were taken to aid in keeping the temperature constant. 

 With three tanks, three temperatures could be run simultaneously. 



The moisture content of the soil, in all of these trials, was also con- 

 trolled. Livingston auto-irrigators 12 were used, two in each jar. The 

 maximum water-holding capacity of the soil was determined by the 

 Hilgard 1-centimeter-column method. 13 Two series of moistures were 

 maintained throughout the trials. In order to keep the moisture content 

 of each culture vessel comparable with the others in any one moisture series, 

 half of the culture vessels in one tank were connected with half of those 

 in each of the other tanks, with but one intake valve for that series. The 

 other halves of the jars in the respective tanks were similarly connected, 



12 Livingston, B. E. A method for controlling plant moisture. Plant world 11:39^0. 1908. 



Hawkins, Lon A. The porous clay cup for automatic watering of plants. Plant world 13:220-227. 

 1910. 



Livingston, B. E., and Hawkins, Lon A. The water-relation between plant and soil. Carnegie Inst. 

 Wash. I'uhl. 204:3^18. 1915. 



" Hilgard, K. W. Soils, p. 209. 1911. 



