446 



THE AMERICAN NATURALIST 



[Vol. XL 



the first shock the cell begins to recover until the permeability is 

 again equalized and pressure and flow cease. When the perme- 

 ability is equalized at a high temperature, as well as at a low one, 

 suction ensues. Hence the suction so often observed during the 

 latter part of the thaw period. This suction may be partly due 

 to the pressure having expelled part of the sap from the outer 

 layers down toward the root or up toward the branches. Then 

 after the restoration of equal permeability the tendency to equalize 

 with the suction of the inner wood would tend to cause some 

 suction in the outer trunk. The suction during cold nights may 

 be partly due to the inner ends of the pith-ray cells being warmer 

 than the outer thereby causing unequal permeability and con- 

 sequent pressure in the reverse direction. But I believe that the 

 wood of maple is normally under suction at this period, as is that 

 of so many other trees, and that the return to suction either at a 

 high or low temperature is merely a return to the normal. There 

 is probably always some increase in permeability whenever one 

 end of the cell is warmer than the other, hence pressure does not 

 drop to zero until after the temperature has become equalized 

 throughout the trunk each day; and on succeeding days, even if 

 there has been no frost, the morning rise of temperature causes 

 some pressure because of the same uiie(jii;il \v;iniiiiii:. It seems 

 reasonable to suppose that some sugar pa-^^cs into the vc:.sels at 

 all times during the period of starch conversion, otherwise the 

 concentration in the cells would become very great. The passage 

 is probably less at low temperatures and greater at high temper- 

 atures. The warmer end of the cell is therefore always the one 

 toward which flow is directed. Maple probably differs from other 

 trees having starch stored in the pith rays mainly in (a) the sen- 

 sitiveness to temperature causing marked unequal permeability at 

 the two ends of the cell and (b) the spasmodic effect of this stimulus 



llw piofophisui of all -ugar maple trees is probably not equally 

 sensitive. For instance, tor a number of years 1 have observed 

 a tree which iiowcd comparatively little sap although this was 

 unusually sweet, flowed les> vigorously on a good sap morning 

 than most trees, and continued flowing after the other trees had 

 ceased during a warm spell. I suspect that here the protoplasm 



