52 TREES AND TREE-PLANTING. 
closing the hole the pressure rose to its former level in 
ten minutes. A stop-cock having been inserted into the 
hole, it was found that the communication between it 
and each of the two gauges was almost instantaneous ; 
proving that the tree was entirely filled with sap and 
exerted its pressure freely in all directions. This sap- 
pressure continued to increase until May 11, when it 
represented a column of water 84.77 feet high—probably 
the highest pressure of sap ever before recorded. This 
pressure gradually decreased until May 27, when the 
lower gauge indicated zero. The suction manifested by 
the birch was very little, never exceeding nine feet of 
water, and continued for but a few days. 
To determine whether this pressure was due to the 
vital action of the roots alone, a root was followed for a 
distance of ten feet from the tree, and then, one foot 
below the surface, cut off. To this detached root, one 
inch in diameter, a gauge was attached, April 26. The 
pressure became immediately evident, and rose, with 
slight fluctuations, until noon of April 30, when it in- 
dicated a column of water 85.80 feet high. The origi- 
nal experiment of applying a gauge to the grape-vine, 
first tried by Rev. Stephen Hales, of England, one hun- 
dred and fifty years ago, was now repeated, May 9, and 
on the 24th showed a pressure of 49.52 feet of water— 
six and a half more than was observed by Hales. 
The peculiar features of the vine-sap are its lateness 
in the season, its apparent independence of the weather, 
its moderate and uniform rise to its maximum, its grad- 
ual decline to zero without marked fluctuations, and its 
almost unvarying suction of from 4.5 to 6.5 feet of wa- 
ter between June 20 and July 20, when the observations 
ceased. 
The general indications of the mercurial gauge seem 
to show that the flow of sap is caused by the absorbent 
power of the roots forcing water into the tree, and as, 
even in the maple, the sap rarely rises more than twenty 
