MODEL TESTS OF VAPOUR MOVEMENT IN SOIL 71 



These results, however, are not simply due to the inversion of the moisture 

 gradient or to low moisture content in the basal containers, although this 

 must be taken into consideration, too. In this case the temperature gradient 

 proved to be too weak. 



Temperature gradients of 30°C and more are well able to force water 

 out of even such dry soils into colder parts, regardless of their water content. 

 This was achieved in our experiments by refrigerating the apical tubes 

 while the bottom of the apparatus was heated by hot water. 



If the water bath v/as omitted, leaving the apical containers enclosed in 

 their ice-wrap, a temperature gradient of about io°C appeared, and this 

 too was followed by the same results in moisture movement as described 

 above. 



By an inverse temperature gradient caused by cooling the lower tubes, 

 the water vapour was caused to move downward, and the upper tubes 

 lost water in favour of the lower ones. 



2. Model Tests on the Ecological Effect of Vapour Movement in 

 Soil upon Plants due to Temperature Gradients 



The tests described in the preceding section were to provide information 

 on the approximate range of vapour transfer in soil due to the temperature 

 gradients. It should be tested now whether that vapour movement might 

 be of practical use to the water relations of plants. 



For the purposes of these tests the apical tubes containing pea-seedhngs 

 were used. After the soil surface had been sealed with petroleum jelly-vasel- 

 ine, water could be lost from these containers only by transpiration of the 

 test plants. This transpiration loss was registered by weighing the apical 

 tubes at the end of each day. The actual loss of water by transpiration 

 should be somewhat higher than the estimations made in this way, because 

 these data are affected by the gain of substance due to daily photosynthesis 

 and growth. The error, however, may be neglected since it does not alter 

 the general results of the experiments. 



In this series of tests temperature and temperature gradient as well as 

 the soil moisture content had to be varied. The most typical variations will 

 be described in the following : 



(a) Equal, Sufficiently Abundant Soil Moisture in Both Tubes 



This is the simplest case. The soil in both tubes held about 10% water. The 

 bottom of the basal tube was heated again by the water bath; temperature 

 gradient could be maintained at io-i5°C through every day. Within the 

 6 



