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THE AMERICAN NATURALIST 



[Vol. XL 



bright mornings following cold nights, sap flowed in considerable 

 quantity from the stubs where branches had been pruned from 

 maple shade trees on the Cornell campus. In the daily papers 

 were reports of sugar having been made during that same month. 



The best sap days are those in which a bright sunny morning 

 with rising temperature follows a frosty night. The flow is greatest 

 early in the morning, decreases gradually as the day advances, 

 and ceases altogether during cold nights. It is not a daily perio- 

 dicity, however, since on many days no sap flows, while again the 

 flow may continue all night. It seems necessary that the rising 

 temperature should cross the 0° C. line in order that there should 

 be a good "run" of sap. If the temperature remains for several 

 days above this point or for several days below it, the flow will 

 rapidly diminish, and in from 24 to 36 hours cease altogether. The 

 trees will then "dry up" and have to be retapped, even though the 

 temperature fluctuates considerably. Hence comes the popular 

 belief that the roots must freeze at night in order to obtain a good 

 "run" the following day. Depending upon the weather, there- 

 fore, the sap flow is usually broken up into periods known as 

 "runs." The swelling of the leaf-buds marks the end of the flow, 

 or "season." If the day be too bright after the frosty night, the 

 flow is apt to start briskly and soon lessen or cease, or if the wind 

 be high the flow is soon checked. If the sky be overcast and the 

 air has warmed slightly, a satisfactory run is likely to ensue. 

 Alternate freezing and thawing, — moderately warm days preceded 

 by freezing nights, — are the ideal meteorological conditions which 

 promote the flow. Other things being equal, the How is usually 

 greater on southern exposures, since there the temperature ex- 



A manometer attached to a bleeding maple tree shows that a 

 considerable pressure exists within the tissues of the wood. It is 

 this pressure which causes the outflow of sap, and which is the 

 primary phenomenon to be considered. During the best sap days 

 the pressure may rise as high as 6.5 to 10kg. per sq. in., but is usually 

 less. The pressure is highest on warm sunny mornings after a 

 frosty night, and rises very raj)idly after the first sunlight strikes 

 the tree, so that on ordinary bright sap days it has reached its 

 maximum at nine or ten o'clock. After that it gradually decreases 



