16 



WATER-RELATION BETWEEN PLANT AND SOIL. 



centage moisture content for the given day, show the percentage 

 variations in moisture content on the basis of the average. 



Since the same soil was employed in all of our cultures, it follows 

 that the data of moisture content are here comparable, when thus 

 calculated to the dry weight of the soil as basis, although it is of course 

 evident that such values would not be comparable if different soils 

 were involved. 



The essential data in this connection, for two cultures of each of the 

 three species used, are given in table 1. 



Table 1. — Maximum plus and minus variations in soilymoisture content for a period of S4 

 hours, of cylinders of soil with plants, the free soil surface sealed, provided tvith automatic 

 water supply through auto-irrigalors. 



'On basis of weight of dry soil. 



From the data of table 1 it appears that the maximum plus and minus 

 variation from the mean soil-moisture content did not attain the magni- 

 tude of 1 per cent of the dry weight of the soil mass for either of 

 the two 24-hour periods during which hourly observations were taken. 



Inspection of table 1 shows that the soil-moisture contents of the 

 different cultures altered during the month that intervened between 

 the two 24-hour periods. There was a decrease in all cases, excepting 

 alone that of Vicia B, which exhibited a slight increase of soil moisture, 

 from 16.0 to 16.5 per cent. This might have been expected from the 

 fact that the water column was very much shorter for the second 

 period than for the first, but Vicia A altered in the opposite direction, 

 from 17.8 to 15.1 per cent. No information is at hand for explaining 

 this occurrence. 



In the Coleus and Pelargonium cultures, where the water column 

 was twice as long for the second period as for the first, the decrease in 

 soil moisture was from 16.6 and 18.9 to 15.8 and 16.2 per cent for the 



