WATER-RELATION BETWEEN PLANT AND SOIL. 15 



The dry weights of the plants were probably somewhat greater at this 

 time than at either of the day series. As to probabilities in relation to 

 the plant water content and its variation, nothing can be said, but it 

 will be seen directly that the errors thus introduced must have been 

 very small. 



When the cultures were discontinued the general tare was determined 

 for each, including the weight of the cylinder and seal, of the irrigator 

 cup, stopper, and glass tube (all filled with water, as in operation), 

 and of that portion of the rubber tubing connection (also filled) which 

 had been supported by the balance in the regular weighings of the 

 cultures. The roots were removed from the soil as far as possible and 

 their green weight, together with that of the corresponding tops, were 

 regarded as the plant weight above mentioned, and this was also added 

 as part of the general tare. The dry weight of the soil was also deter- 

 mined for each culture. From the total weight of the culture for any 

 observation was subtracted the general tare plus the dry weight of the 

 soil, and the remainder was assumed to be the weight of the soil 

 moisture. 



If the dry weight of the plant varied, the error thus involved should 

 introduce a relatively very small error in the magnitude of the dry 

 weight of the soil, the latter weight being very large in relation to the 

 former. When the water content of the plant was high om* calculated 

 soil-moisture content must have been too great; when the plant con- 

 tained less water the calculated content of the soil must have been too 

 small. The dry weight of the soil ranged from 1,875 grams to 1,977 

 grams in the various cultures, and it is readily seen that no considerable 

 error was introduced in this connection. The water content of this soil 

 mass had a value ranging from about 150 grams to about 375 grams. 

 Thus possible fluctuation in water content of the relatively small plants 

 could account for but a small error in any case. 



In the actual calculations, the results of which are about to be pre- 

 sented, the average weight of the moist soil during each of the two 

 24-hour periods was determined for each culture. From this was 

 subtracted the corresponding dry-soil weight, the resulting difference 

 being the average weight of the contained water for the period in 

 question. This was divided by the dry-soil weight and the quotient 

 multiphed by 100 to give the average soil-moisture content, in per- 

 centage on the basis of the dry weight. The greatest plus and minus 

 variations from the average weight of the culture are considered as the 

 maximum variations in water content, stated in grams. These quan- 

 tities, divided by the corresponding dry-soil weight, and the quotient 

 multiplied by 100, in each case, furnish the maximum plus and minus 

 variations, expressed as percentage on the basis of dry-soil weight. 

 The maximum plus and minus percentage variations in soil-moisture 

 content, divided by a hundredth of the corresponding average per- 



