No. 474] 



SAP FLOW IN MAPLE 



415 



dicating 6.8 kg. pressure caused a drop of 3.2 kg. before a stop- 

 cock could be inserted. 



The sap flow varies in quantity with the pressure, so that a 

 separate discussion of its characteristics is scarcely necessary. Some 

 points, however, may be noted. On good sap days the quantity 

 is often great, being as much sometimes as 12 liters in 10 hours. In 

 some exceptional cases a flow of 20 liters per day has been re- 

 corded. Usually the flow is much less, 10 to 12 liters a day being 

 an average flow for a moderately good sap day. According to 

 the Vermont Bulletin it seems probable that a high registered 

 pressure is not absolutely necessary- to a good flow of sap, but 

 that less pressure with longer duration will give equally good 

 results. The rate of flow seems to depend also upon the amount 

 of sap present in the wood around the tap-hole as well as upon 

 the pressure behind it. 



In general the flow is greatest near the ground. Clark inserted 

 a spout at the usual height into a healthy maple which had never 

 been tapped, and fifteen meters above this another spout was set 

 into the trunk where it was 13 cm. in diameter. In addition, a 

 limb 10.6 meters from the ground was also cut ofT where it was 

 2.5 cm. in diameter. In several hours the lower spout had bled 

 2.7 kg. of sap, the limb 56.7 gm., and the upper spout not a drop- 

 Similar experiments with other trees gave like results. Both 

 Clark and the A'ermont workers found the down flow, as well as 

 the down pressure, greater especially at the ordinary height of 

 tapping. At both places, too, it was found that if an incision is 

 made into a tree the sap will flow from the upper side of the cut 

 and not from the lower unless late in the season. In late spring 

 the flow is usually from both surfaces. In other words the flow 

 is down from above in the maple, not up from the roots. Clark 

 found that a severed tree would bleed profusely from the cut 

 surface while the stump remained nearly dry. In Vermont it was 

 found that in many cases severed twigs that started to bleed very 

 early in the season frequently ceased before the flow from the tap- 

 hole diminished very much. Lithium chloride inserted in the 

 tap-hole showed that at times, at least, sap under pressure 

 moves in the vicinity of the outlet hole at the rate of from 5 to 



