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JOHN H. EHLERS 
larger — 0.09 mm. The wires were carefully soldered, and all junc- 
tions having rough or thick joints rejected. A thin coating of shellac 
was given the junctions to protect them against oxidation and any- 
possible action of the leaf juices. 
5. Method of Insertion into the Leaf. — The leaves of Pinus laricio 
austriaca grow in bundles of two. The leaf in cross section represents 
roughly a semi-circle in outline about 1.5 mm. in its shortest diameter. 
To insert the junctions into the leaf at temperatures ranging from 
0° to — 17° C. was attended with considerable difficulty and many 
junctions were broken, especially in the beginning of the experiments. 
The procedure was to thread the lead wire through a very fine steel 
needle and draw the wire through the leaf until one of the junctions 
was embedded in the tissue. To prevent too great a loss of heat by 
conduction along the lead wires, as much of the wire as possible should 
be embedded in the leaf tissue. Drawing the wire through at right 
angles to the long axis of the leaf would not, therefore, give the best 
results. Drawing the wire through the leaf lengthwise was found 
impracticable because of the firmness of the tissue. Attempts to do 
this usually resulted either in the splitting of the epidermis, or the 
breaking of the leaf. Placing the junction and lead wires between 
two leaves fastened together flat surface to flat surface did not give 
good results. The method finally adopted was to fasten the two 
leaves together by means of two single turns of very fine wire placed 
about I cm. apart, the wire covered with white insulation. The 
leads were drawn through both leaves at an angle of 45° with their 
long axis. Care was observed to leave the junction embedded in the 
leaf toward the sun and as near its center as possible. This gave a 
contact surface of about i mm. for the advance lead wire and from 
3 to 4 mm. for the copper — the better conductor of the two. The 
second junction was left exposed to the air and shaded from the sun's 
rays by a wooden screen. The junctions, therefore, registered directly 
the differential temperature between leaf and surrounding air. 
6. Position of the Leaf. — The leaves of the pines assume no definite 
position with relation to the sun's rays. In all experiments, unless 
otherwise stated, a leaf normal to the sun's rays, or as nearly normal 
as could be found, was selected. Two sets of junctions, each in a 
different leaf, were constantly in use — the one a check upon the other. 
A double-pole double-throw switch on the table beside the operator 
made it possible to read the temperatures of the two leaves in rapid 
