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



(i) When soil moisture has decreased below field capacity Hquid water will 

 be conducted through soil at only an extremely slow speed. 



(2) If roots have dried out a certain soil volume available, the lost soil 

 moisture will be replaced so slowly from the neighbouring parts of 

 soil that usually the roots elongate much more rapidly, thus actively 

 invading new water sources (Walter, 195 1 ; Kausch, 1955 ; Veihmeyer, 

 1956). 

 Apart from this group of contributors there are those who consider the 



vapour movement in soil under temperature gradients to be of practical 



importance to plants (Brawand & Kohnke, 1952; Lebedeff, 1927, 1928; 



Smith, 1934, 1943); in particular Trenel (1949, 1950, 1954, 1955); Trenel 



et al. (1958). The main points of view of this group are as follows: 



(i) Plants can be kept at fuU turgescence over several weeks, even months, 

 getting the necessary moisture by no other means than a vapour- 

 saturated atmosphere. Plants cultivated in this way may even grow, 

 flower and bear fruit if provided with a suitable amount of minerals. 



(2) If open Petri dishes or similar glass containers are mounted in hori- 

 zontal tunnels driven at different levels into a soil profile, condensed 

 water wiU accumulate in these dishes on warm days (Trenel, 1954, 

 measured about 30 mm per year). 

 Those experiments, however, did not deal directly with the problem of 



whether the plants are able to use the vapour condensation in soil, caused by 



temperature gradients. 



On account of the methodological difficulties comphcating experimental 



work in the field a series of model tests was performed to produce some 



contributions to this subject. 

 Work was done in 1957 in the Field Station of the Botanical Institute at 



Potsdam-Sanssouci, Germany. Technical performance was done by a 



student, Christa Giesche, using experience achieved in our laboratory 



from previous investigations made on similar subjects under direction of 



Dr. Lore Steubing. 



II. MATERIAL AND METHODS 



For the purposes of this subject an experimental set-up had to be developed 

 allo^ving the changes of soil moisture to be traced during the test without 

 disturbing the test plants. This was achieved in satisfactory manner by the 

 following equipment (Fig. i) : 



Each apparatus consisted of two glass tubes, 4-5 cm wide and 10 cm long, 

 one put upon the other after having been prepared before in different ways. 



