No. 474] SAP FLOW IN MAPLE 421 



Sachs says: "If we assume that a stem and root has reached an 

 even temperature between 0° and 4° R, and then suddenly a marked 

 warming of the air occurs, at first only the crown and the stem are 

 warmed, the thinnest twigs first of all; a part of their water is 

 forced back into the thicker twigs, these in turn are warmed and a 

 portion of their water is forced back into the cold trunk, which 

 is warmed most slowly. Even this at length becomes warm and 

 soon water is forced into the root. If now after the roots had 

 become warm the trunk and top were to become cold through a 

 sudden fall of temperature in the air, then a flow would occur 

 from the roots toward the stem and from the stem toward the 

 branches and so on." He was not sure whether this was the sole 

 factor in the sap flow of the maple, birch, etc., in winter.^ 



But is the gas-expansion theory really capable of accounting 

 for the pressure observed in the maple tree during the sugar 

 season? In the Vermont report, a pressure of 24 lbs. (10.8 kg.) 

 per sq. inch was given in one instance, while pressures of 10 to 15 

 lbs. (4.5 to 6.8 kg.) were frequent. The change in external tem- 

 perature was in these cases about 5° to 8° C, which would mean 

 a change of perhaps 3° to 4° within the tree; or, if the sun shone 

 directly upon the tree, perhaps there would be a rise of from 5° 

 to 10°, especially in the outer wood and twigs. G&s pressure in- 

 creases at the rate of .,}.^ for each degree of rise in tem{)erature. 

 A ().8 kg. pressure (atmosf)heric pressure per sq. inch) would 

 thcicfoie c(iual s In ^^itj, a rise of one degree. A ri^e of 

 fic.iii -. lo I().N^M.> lui. Mll)p()std,^^()uld increase the pres- 

 sun> ..niv iVuM, ko-. ,o r, ka-., a verv small amount com- 

 pared witli the 4.5 to kg. pressure actually obtained. The 

 Vermont {)eople are justified, therefore, in asserting that gas 

 expansion cannot possibly account for the pressure observed in the 

 sugar maple. 



unci Ausflu.ssgesdnvindigkeit von Safteii IcIh ikIci ril:ni/cr, / , 

 p. 97, 18G2). 



