208 Moisture Equilubrvum [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. 8. Ho~mzs 
While the auto-irrigator devised by Livingston* has been 
employed by several writers,” 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 
* Livingston, B. E., “ A method of controlling plant moisture.” 
Plant World \1: 39-40. 1908. 
? 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 O. 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. 
