No. 474] 



SAP FLOW IN MAPLE 



413 



toward nightfall. The time of highest pressure does not there- 

 fore coincide with that of highest temperature, but often precedes 

 the latter by several hours. The maximum pressure on a good 

 day usually occurs about one and one half hours after commencing 

 in the early morning. If the following night is also cold, then all 

 through the night a moderate suction will obtain again, to be 

 followed by a similar abrupt rise the following morning. In one 

 case, Clark read on the manometer at 6 a. m. a suction sufficient 

 to raise a column of water 7.89 meters high, while as soon as the 

 sun shone upon the tree the mercury suddenly began to rise so 

 that at 8.15 a. m. the pressure outward was enough to sustain a 

 column of water 5.63 meters in height, a change represented by 

 more than 13.5 meters of water. On another morning the change 

 was still greater representing 14.45 meters of water. If the 

 night remains warm preceding a thaw, the fall of pressure will 

 be much less rapid, and a moderate pressure may continue all 

 through the night gradually vanishing during the following day. 

 After several days of thaw, suction may obtain most, if not all of 

 the time. If the temperature remains below freezing, suction 

 may exist for several days until the weather warms. There is 

 much to indicate that the normal condition in the maple at this 

 period is one of suction. 



A rise of only a few degrees will often cause very great pressure 

 if the rise passes the zero point Centigrade. On the other hand 

 there may be considerable fluctuation in temperature without 

 great fluctuation in tension. This happens when the temperature 

 does not cross the zero line. The pressure fluctuations are greatest 

 early in the season. During the day, pressure foixrs into the 

 tap-hole all the sap located in the adjacent tissue. The suction 

 wiiich (Misues on freezing nights possibly draws more sap into 



uj) auain. The entrance of air is hindered by the impermeability 



of bright sun and clouds, the gauges fluctuated very markedly. 

 Two pressure gauges were placed at a distance of twenty feet 



