216 BOTANICAL GAZETTE [MARCH 
None of the substances mentioned give good results when used 
alone. Paraffin (45°) stretches, petrolatum creeps, and beeswax, 
tallow, and the higher paraffins crack. Except in the case of the 
soft paraffin, which stretches, the plants show no difficulty in pene- 
trating the wax cover, even when a wax as hard as that used in 
making the ordinary commercial phonograph, records is used as a 
seal. 
During the winter in the greenhouse the paraffin-petrolatum 
mixture gives excellent results even when left in direct sunlight. 
However, during the warm portion of the year direct sunlight is 
likely to melt this wax, and in this way break the seal and cause 
damage to the plants, due to the wax creeping over the plant sur- 
face. The: beeswax mixtures have proven best during warm 
weather. A mixture of 10-30 per cent beef tallow with beeswax, — 
or of 8-12 per cent of petrolatum with beeswax, has proven an excel- 
lent material both in greenhouse and for out-of-door work. 
Modelling clay has also been used to seal the pots, but it is not 
so easily applied as the wax, and is not suitable for use with delicate 
seedlings. 
Experimental error 
In determining the non-available moisture in a given soil, some 
variation will be found in the results obtained from the individual 
pots. This variation is probably due in part to the lack of uniform- 
ity of the soil in the different pots, but, mainly to the fact that im 
some pots the roots are distributed through the soil mass much 
more uniformly than in others. When the root distribution 1S 
defective, the mean distance through which the soil moisture must 
move through capillary action is relatively greater. Since capil- 
lary movement is very slow in soils that are approaching their 
non-available moisture content, the portion of the soil not pene- 
trated by roots would have a somewhat higher moisture content. 
The non-available moisture determination in the case of an impet- 
fect root distribution would consequently be somewhat too high. 
On the other hand, errors arising from the distillation of water 
to the walls of the pot would result in giving a non-available 
determination below the true value. : 
Some uncertainty also arises in connection with the determina. 
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