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THE AMERICAN NATURALIST [Vol. XL 



and one half hours after pressure begins in the morning. From this 

 time on pressure falls gradually to the zero point. The highest 

 pressure occurs, therefore, several hours before the time of maxi- 

 mum air temperature for the day, and the maximum temperature 

 within the tree would be still later. Still the most abrupt rise is 

 no doubt early in the morning when the sunlight first falls upon 

 the tree, while the subsequent daily rise must be much more 

 gradual. It might, therefore, be inferred that later in the day the 

 percolation of sap into the fibers is sufficiently rapid to offset the 

 expansion after the first abrupt rise. 



When the temperature again falls below the freezing point, 

 water would be drawn back into the vessels from the fibers and 

 from more distant parts of the tree whither it had been forced. 

 In many cases, as seen in the Vermont Bulletin, suction is greatest 

 at first, but gradually decreases if the cold persists for some time. 

 Suction during cold nights might be due either to the presence of 

 a normal two or three pound suction in the tree at this season, or 

 to the difficulty which the fluid that had passed into the wood 

 fiber encountered in going back through the walls. When the 

 temperature remains high, for a long time little pressure occurs 

 though the fluctuations in temperature may be great. Only a 

 previously low temperature insures a good run when the mercury 

 again rises, and it is better if the cold endures for several days. 

 A cause of this might be that at the high temperature, air creeps 

 into the vessels from the expanded gas in the various tissues so 

 that fluctuations are no longer transmitted. During the con- 

 tinued cold the air would pass back to the older cells thus leaving 

 once more a solid column of water. 



The objection that the sap occupies its least volume at 4 degrees 

 and therefore at a temperature above that at which pressure begins, 

 is invalid because only pure water Ix^liaves thus. With concentra- 

 tion of solution this density point t'alU ninch .nnn- nipidlv than the 

 freezing point, becomes Irss mark.vl, iind m.,.,, l.<-<-,),„es ideiuical 



centration it u..ul<l either have disuppeure.l ('..tireiv ,.r at lea.st have 

 fallen to 0° C. wIhmi it could no longer be use<l as an objection. 



It is po.ssible, therefore, to exj)lain tiie extreme pressure and 

 many fluctuations peculiar to the maple by this theory, but it has 



