SOIL MOISTURE AND TO EVAPORATION. 



67 



before wilting. In Experiment IV wilting occurred on the day follow- 

 ing the end of the record and the rate is probably not far from correct, 

 while in Experiment V the plant began to wilt only 6 hours after the 

 weighings were discontinued and therefore the rate is very nearly cor- 

 rect in this case. In Experiment XII wilting was manifest 3 days 

 after the end of the record, so that here again the rate given in the table 

 is probably too high. The data at hand do not represent a sufficient 

 number of plants to warrant a critical study of the relations existing 

 between the transpiration rate and the amount of moisture in the 

 soil at the time of wilting. This is a field for another investigation. 

 The problem involves not only the tenacity with which the soil withholds 

 its water from the plant, but also the rate of water movement through 

 the soil from one region to another. 



TABLE XVI. Moisture Contents of Soils in which Mesophytes Wilted. 



Several other determinations of the moisture content of the soil 

 when wilting occurred were made without transpiration records. Cab- 

 bage seedlings wilted July 18 with a moisture content in the soil of 

 11.10 per cent. Three different soil samples taken August 15 from the 

 root systems of Boerhavia plants which were beginning to wilt in the 

 open showed moisture contents of 6.40, 6, and 6.74 per cent, while 

 another sample taken from the root system of a plant which was still 

 vigorous showed a moisture content of 13.6 per cent. 



In February, 1905, a number of well-grown potted plants were taken 

 from the greenhouse at the Hull Botanical Laboratory of the University 

 of Chicago and placed in one of the laboratory rooms, where they were 

 allowed to stand without addition of water to the pots until wilting 

 occurred. When this occurrence was noticed soil samples were taken 

 from the midst of the root systems and their moisture contents were 

 determined. The names of the plants and the moisture contents which 

 corresponded with the incipient wilting are given in Table XVI. The 

 soil was a sandy garden soil, containing considerable humus. As 



