78 W. R.MULLER-STOLL ANDG. LERCH 



(e) Inverse Temperature Gradient 



If the temperature gradient was inverted by cooling the basal tubes, w^ater 

 vapour moved dov^nward, causing the relations shovv^n by Fig. 7. 



This time the soil within the upper containers lost more water the higher 

 the initial soil moisture content. Consequently water moved downward, 

 and increased the weight of the basal tubes. Due to this additional loss of 

 water from the upper tubes the test plants wilted on the fourth day of the 

 experiment, whereas the control plants were kept fresh up to the end of the 

 test, after 7 days. 



IV. DISCUSSION 



The data of our model tests proved that under the experimental conditions 

 chosen here the vapour movement and condensation in soil due to tem- 

 perature gradients had practical importance to the plants. 



As long as the basal soil columns were heated and thus the temperature 

 gradient was maintained water vapour moved upward into the apical 

 tubes, and the test plants grown here always wilted several days later than 

 in the control containers with uniform temperature conditions, where the 

 roots had nothing but the moisture of the apical tubes available. 



The quantity of water moved in this way depends, at least, upon three 

 variables : 

 (i) Range of temperature gradient. 



(2) Temperature levels. 



(3) Soil moisture content. 



Apart from these, the duration of the temperature gradient and the specific 

 texture of the soil concerned will doubtless affect the vapour movement, 

 too. 



The tests performed by relatively simple means do not yet allow a mathe- 

 matical formulation of the influence of the variables, but a few relations 

 stand out quite distinctly. 



By the same temperature gradient considerably more water is extracted 

 from a greater soil moisture, at best not much below field capacity (see 

 Smith, 1934, 1943), than from a lower moisture content. Likewise at high 

 temperatures more water is translocated than at low temperatures. 



Most effective is an increase of the temperature gradient itself. The 

 extremely high gradient of 30°C was able to extract water even from a 

 very dry underlying soil and to force it upward into moister layers of 

 soil, a procedure that had been impossible under temperature gradients of 

 merely io°C. 



