RESEARCH METHODS IX STUDY OF FOREST ENVIRONMENT. 167 
the same time permits the entrance of air, which may be obtained 
by the roots through the walls of the pot, insuring their mainten- 
ance in a healthy condition. 
For the best distribution of moisture in the soil, it is desirable 
that a second tube, or a tee connection on the longer one. should be 
placed so as to extend about an inch below the soil surface. Since 
there will be some escape of vapor from each of these tubes, a pot 
similarly prepared, but containing no tree, should be run as a 
measure of such loss, under the various conditions to which the 
trees are exposed. 
The weight of the can. pot. and glass tube is first obtained. The 
seedling to be potted is then weighed with the minimum of exposure 
to the air. The seedling is then placed in the can, which is filled 
with moderately dry soil, and the weight of the whole is obtained 
immediately after potting, following which water may be given to 
the plant. Having now the weight of the air-dry soil which has 
been placed in the pot. its net oven-dry weight may readily be com- 
puted, after drying small samples of the same soil; and from this 
net weight may be calculated the amount of water which the pot 
should contain at all times to maintain a moisture equal to 50 per 
cent, let us say. of the saturation capacity of the soil. Once the soil 
i.^ well settled by watering, the top of the can is sealed with a mix- 
ture of paraffin and vaseline. A measured amount is used, so that 
the weight of this substance may also be included in the total weight 
which the outfit should show at the desired moisture condition. 
Knowing the weight which the outfit should have at a certain 
moisture condition, the simplest method of measuring the transpira- 
tion is to put the can on one side of the scales and the desired weights 
on the other side, and to inject water from a burette until a balance 
is obtained. The water may alternately be injected through the long 
and the short glass tubes. 
The transpiration results will be most expressive if given in terms 
of transpiration per unit of leaf area ; but since, with coniferous seed- 
lings, the determination of leaf area with any precision is next to 
impossible, the plan of computing the loss per unit of weight of the 
plant may be considered. 
Where a number of plants, even though of the same species and 
grown under the same conditions, are to be placed in potometers for 
transpiration study under a variety of field conditions, the plants 
should by all means be calibrated under the same conditions before 
being distributed, since extremely great variations in individuals 
seem to be inherent. 
