214 
K77>, 
[MARCH 
BOTANICAL GAZETTE 
In case this is not feasible, the pots can be immersed in a barrel 
or large tank of water, in which a constant circulation is maintained. 
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Fic. 2.—Showing tank for preventing sudden changes in the temperature of the 
soil used in non-available determinations; the propeller in the central tube causes 4 
constant circulation of the water around the pots. 
The most satisfactory form of stirring device that we have used 
consists of a small propeller in a tube (fig. 2). The upper end of 
the tube carries a yoke which supports the bearing of the propeller 
