208 Moisture Equilibrium [406 



method of measurement of light here given, although it is only 

 a rough approximation and depends on the heating effect of 

 the sunshine, has been shown, as a matter of fact, to give 

 numbers rather definitely correlated with plant growth. It 

 has been found, for instance, that the amount of dry sub- 

 stance produced per unit of leaf area in young soy-bean 

 plants decreases from the beginning to the end of the growing 

 season, in a manner which generally parallels a corresponding 

 fall in the light intensity values as determined in the manner 

 described above. 



MOISTURE EQUILIBRIUM IN POTS OF SOIL EQUIPPED 

 WITH AUTO-IRRIGATORS 



By F. S. HOLMES 



While the auto-irrigator devised by Livingston 1 has been 

 employed by several writers, 2 for maintaining uniform mois- 

 ture conditions in potted soils, the details of adjustment re- 

 quired by this device, for different soils and for maintaining 

 different moisture contents, remain still to be worked out. 

 In order to throw some light upon this general question, a 

 study of three different soils was undertaken to determine 

 the relation between the equilibrium point of the soil-moisture 

 content and the number of irrigator cups employed. 



One soil was a medium-fine white sand, one was a light 



1 Livingston, B. E., " A method of controlling plant moisture." 

 Plant World II: 39-40. 1908. 



2 Hawkins, Lon A., " The porous clay cup for the automatic water- 

 ing of plants." Plant World 13: 220-227. 1910. Transeau, E. N., 

 " Apparatus for the study of comparative transpiration." Bot. Gaz. 

 52: 54-60. 1911. Livingston, B. E., and Lon A. Hawkins, "The 

 water relation between plant and soil." Carnegie Inst. Wash. Pub. 

 204: 5-48. 1915. Hibbard, R. P., and 0. E. Harrington, "Depres- 

 sion of the freezing-point in triturated plant tissues, and the mag- 

 nitude of this depression as related to soil moisture." Physiol. Res. 

 I: 441-454. 1916. 



