SAP PRESSURE IN 'THE BIRCH STEM 
“PART I 
H. E. MERWIN AND HowaARD LYON 
(WITH FIVE FIGURES) 
During the seasons of 1902 to 1904 sap pressure observations were 
made on several kinds of trees in the vicinity of Oneonta, in central 
New York. Birches and maples illustrate the two extreme types of 
sap pressure phenomena. Our observations on the maples are in 
accord with those of other observers, especially as set forth by JONEs, 
Epson, and Morse.' Sap pressure in the birches has not been 
studied much hitherto, except as incidental to other studies. 
We found that glass tubes of small bore filled with mercury made 
very sensitive pressure gauges, especially if the tap hole in the tree 
and the connecting tubes were filled with water or sap when the gauge 
was attached. When pressure was negative (suction), a bulb tube 
was sometimes arranged to allow water to flow into the tap hole, and 
to catch the gas which escaped. Gas in the tap hole when pressure is 
negative causes disturbing capillary effects. Gas in the tubes has a 
damping influence upon the gauge. 
Characteristics of sap pressure in the birches 
No sap will flow from tap holes in the stem of the birch or ooze from 
cut twigs till the ground has thawed considerably in the spring. 
It is not necessary, however, that the air temperature be continuously 
above the freezing point before pressure becomes high. From April 5 
to April 22, 1904, we have recorded seven nights in which the tempera- 
ture was below freezing; yet on April 5 a positive tension of 44.3°™ 
was observed in a yellow birch (Betula lutea); April 9, 87°™ in a 
black birch (Betula lenta); April 18, 3 5°™ ina black birch. F reezing 
nights were often accompanied by negative pressure which was 
maintained for a few hours after sunrise. The maximum pressure 
comes about a month after the first decided appearance of pressure. 
The buds by this later time have begun to unfold.. There is at all 
1 Jones, C. H., Epson, A. W., anD Morse, W. J., The maple sap flow. Vt. 
Agric. Exper. Sta. Bull. 103. 1903. 
Botanical Gazette, vol. 48] L442 
