No. 474] 



SAP FLOW IN MAPLE 



445 



increasing stimulus, but as an abrupt one. As the critical temper- 

 ature is reached the mechanism of stimulation is perhaps set off 

 all at once, so to speak, and the maximum permeability is reached 

 very soon, and consequently the maximum pressure and flow. As 

 the day progresses either the already converted sugar in the cell 

 is exhausted or the membrane gradually recovers its normal con- 

 dition as it recovers from the stimulus. It seems more likely 

 that the recovery is not due to the exhaustion of sugar content 

 because on succeeding warm days without freezing nights there 

 is still evident considerable fluctuation with temperature showing 

 that some sugar is still there. Then again, if the membrane 

 remained unequally permeable, the small amount of sugar con- 

 version that does constantly occur would tend to maintain a 

 constant though slight pressure until by a fall to 0° C. the mem- 

 brane became again equally permeable ; but instead suction usually 

 soon occurs. It seems much more reasonable that after the abrupt 

 stimulation the protoplasm should soon gradually recover its 

 original condition. The slight fluctuation in pressure that occurs 

 each morning even during a thaw period is prol)ably due either 

 to a recurring but slighter degree of difference in permeability, 

 or to an abrupt increase in sugar production induced by the rising 

 temperature. Since abrupt fluctuation in sugar production suf- 

 ficient to cause pressure is iniprobal)le, the former hypothesis 

 seems the more reasonable. 



There is no reason to believe the conversion of sugar to be 

 otherwise than normal, that is, gradual and constantly [)r()gressi\ e, 

 less rapid when the temperature is low and more rapid when higher 

 according to the normal action of enzymes. I see no reason to 



converting a large quantity of starch into suuar abnipfly at one 

 end of the cell, and thus causing pressure tliroiiuli the uiHM|ual 

 distribution of the solute. It seems to ine more prohaMf that at 



uniformly iinpcrnieatilt^ over it^ eiitirt' surt'acf. ()-nintir prr-^siire 

 is therefore high an.l the cells a.v very tn.-id. A itm" of u-mpvv- 

 ature to the critical point now cans,-s the ahrupt Miinulatin- shock, 

 sugar passes out at the peripheral ends of the cell^. and both 

 pressure and flow become great toward the outer wood. After 



