CAUSE OF SUCTION. 263 



pressure equal to 10.77 feet of water, while, at the same time, 

 the upper gauge showed a pressure of 24.93 feet. On the 

 thirty-first of March, the gauges were all frozen, number one 

 standing at 28.90 feet of water, while number five indicated 

 a suction equal to -26.07, a difference of 54.97 feet. In the 

 case of number one, attached to the trunk near the ground, it 

 seemed that the gauge froze before the body of the tree was 

 much chilled, while, by the sudden freezing of the branches, 

 the sap was abstracted from the upper gauge before the cold 

 had penetrated the coverings sufficiently to freeze it. 



On the nineteeth of April, the upper gauge showed little or 

 no pressure, while the lower one still indicated a pressure of 

 17 feet. This was apparently due to the absorption of the 

 sap from the branches by the expanding buds. 



In view of all the phenomena thus far observed, it appears 

 that the flow of sap from the maple and other species, which 

 bleed only after being frozen, is in no sense a vital process, 

 but purely physical. The sap is separated from the cellulose 

 of the wood by the cold, and, under ordinary conditions, 

 gradually reabsorbed. If, however, the tree be tapped, so 

 that the liberated sap can escape, then it will do so, flowing, 

 as is readily seen to be the case with the maple, most copiously 

 from above. The bleeding is, therefore, a sort of leakage 

 from the vessels of the wood, but this is doubtless increased 

 by the elastic force of the gases in the tree, which are com- 

 pressed by the liberated sap, and this expansive power must 

 be intensified by the increase of temperature which always 

 accompanies a flow. 



This theory explains the fluctuations of the gauges, and 

 accounts for the singular fact that the upper one shows the 

 most pressure and the greatest variations, inasmuch as the 

 branches and twigs would, of course, be most quickly and 

 powerfully affected by the heat of the sun and the tempera- 

 ture of the atmosphere. The pressure of the expanded gases 

 in a tree in a normal condition would facilitate the re-absorp- 

 tion by the wood of the liberated sap. Their contraction by 

 cold would also cause the cessation of the flow from a tree 

 which was running, and produce the remarkable phenomenon 

 of suction exhibited by the gauges at night or during frosty 

 weather. 



