GOOD SAP DAYS. 185 



from several trees with the meteorological observations of the 

 same period, conclusively proves that while the general flow 

 corresponds with the season, rising to a maximum and then 

 declining, yet the daily and hourly flow varies with the 

 weather. The most unfavorable weather is that which is 

 either steadily and severely cold, or uniformly warm and 

 foggy, while the best sap days are such as are bright and 

 warm at mid-day, but preceded by freezing nights. Such va- 

 riations of temperature as aft'ect the flow of maple-sap are 

 most likely to occur when the ground is covered with snow, 

 because the heat of the sun during the day cannot then accu- 

 mulate to moderate the cooling influence of the night. The 

 most probable explanation of the eflect of these alternations 

 appears to be that the outer tissues of the tree are partially 

 emptied of their contents by the contracting influence of 

 cold, the sap being driven into the heart-wood of the trunk 

 and large roots. Meanwhile absorption goes on as usual un- 

 derground, and thus, when the outer layers of the wood are 

 expanded by the heat of the sun, the sap rushes back to the 

 surface and flows abundantly. 



Observations by Biot, in France, un the poplar, and by 

 Nevins, in Ireland, on the elm, seem to show that the sap is 

 thus forced by freezing weather from the outer wood and 

 branches into the heart of trees. 



To determine whether sap would flow from the heart-wood 

 of a sugar-tree, a piece of gas-pipe was sharpened and driven 

 snugly into a hole six inches deep. From this spout the flow 

 was regular and long-continued, Ijut not quite so abundant as 

 usual. This proves that the spring sap does enter and fill 

 the heart-wood aa well as the alburnum under some circum- 

 stances. 



Another tree was selected and a piece of bark five inches 

 long and two inches high was removed from the outer layer 

 of wood, and a piece of sheet-iron driven into the bark below 

 to catch the sap, which flowed very profusely, but stopped 

 very early. The tree, from the heart of which sap was taken 

 as above described, flowed eleven days longer than the sim- 

 ilar one from which only a piece of bark was removed, Imt 

 the latter afibrded twelve pounds more sap than the 'former. 



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