68 W.R. MOLLER-STOLL AND G.LERCH 



by a layer of petroleum jelly-vaseline, and this container was then put with 

 its bottom of Perlon gauze upon the open end of the basal tube filled to 

 the brim with soil. In order to improve the sealing between the two tubes 

 their adjoining ends were enclosed in a piece of folded rubber hose. In 

 addition to it a strip of plaster, 2-5 cm wide, was fitted around the rubber 

 cuffs and held both glass containers together during the test period. 



Thus the soil columns of those two tubes were connected forming a 

 practically uninterrupted unit, merely separated by a Perlon diaphragm 

 which prevented the roots in the upper container from invading into the 

 basal soil column. 



The two containers, however, could be separated at any time during the 

 experiment and individually weighed, thus allowing the control of mois- 

 ture movement in the upper and lower part of the test set-up. 



As a rule, the temperature gradient was induced by a hot-water bath. 

 The bottom of the basal tube was standing to about 4 cm depth in a hot- 

 water basin the temperature of which vv^as controlled by means of an 

 electric heater combined with contact thermometer, relay and electric 

 stirrer. 



Two thermometers with a long and a short base were inserted into the 

 test containers with their mercury bulbs resting in the apical tube about 

 2 cm and in the basal tube 18 cm below the common soil surface. Since 

 the long thermometers piercing the diaphragm prevented the soil columns 

 in their containers from being easily separated, the temperature gradient 

 was checked in all cases by additional closed systems as shown in Fig. ib. 

 They were added to the test containers with rooted plants being thus 

 subject to exactly the same temperature treatment. 



In order to increase the temperature gradient or to cool down the con- 

 tents of the tubes an ice jacket was applied to the containers. For these 

 purposes a hollow cylindrical tin can was constructed the interior convex 

 surface of which fitted tightly around the glass tubes. The can was filled 

 with a mixture of ice and salt thus cooling the top of the apparatus as well 

 as its bottom. 



For technical reasons and in order to approach field conditions the 

 artificial temperature gradient was run, as a rule, from 8.00 a.m. to 6.00 p.m. 

 At night the temperature differences vanished gradually, and on the next 

 morning the soil around the thermometer bulbs in both tubes showed the 

 same temperature of about 2i°C. 



Each experiment, including check tests, was repeated three times at 

 least. Thus the data pubHshed in the following sections each represent an 

 average value of four single observations. 



