RESEARCH METHODS IX STUDY OF FOREST ENVIRONMENT. 165 
piration of the leaf. In order to refer this to a definite standard, it is 
compared with the time required by the strip to make the same change 
over a free-water surface blanketed by one millimeter of air at a tem- 
perature the same as the leaf. The reaction time of each slip must 
therefore be ascertained under standard conditions. The apparatus 
for making these standardizations is described in detail by Livings- 
ton and Shreve (161). 
It would obviously be a great labor to determine the reaction time 
of each slip for every temperature likely to be used ; but this is un- 
necessary, because if the time is determined for one temperature it 
can be easily derived for any other, since it is inversely proportional 
to the pressures of saturated aqueous vapor at the different tem- 
peratures. 
The chief objection to the method is the large personal equation in- 
volved. The fading of the slip is very slow indeed, and no perceptible 
change may occur for many seconds. Practically the system seems to 
work out well and yields much more consistent results than potom- 
eters. It is easily used in the field and in all kinds of sites, and ap- 
pears to be a valuable addition to field research methods. 
Method of excised twigs. 
In measuring transpiration by means of cut twigs placed in 
potometers, the actual amount of water given off per unit of leaf 
surface can be determined. 
The apparatus used consists of a flask closed*with a two-hole rub- 
ber stopper, in one hole of which is inserted a glass tube bent at right 
angles : in the other the twig is sealed. The horizontal section of the 
glass tube is graduated so that it may be used to measure the water 
removed by the twig, and is drawn to a fine point at the end to mini- 
mize evaporation from the end of the tube. The twigs must be cut 
off under water and the ends must be kept continuously wet, or large 
variations in transpiration will occur. The sealing into the potom- 
eters must be very carefully done also, as the least leak will totally 
vitiate results. This method is particularly useful when it is de- 
sirable to determine transpiration of several species or in several sites 
simultaneously. The results tend to be erratic, however, and ac- 
cordingly the determinations should be made in duplicate, at least to 
avoid gross errors. The same twigs can only be used for a short time, 
after which fresh ones must be obtained. 
To determine the leaf area, as must be done to reduce the transpi- 
ration to amount per unit surface, the simplest method is to cut a 
number of pieces of known area with a cork cutter, or preferably a 
centimeter punch, weigh them, and then weigh the residue of the 
leaves and figure their area proportionately. 
