No. 474] 



SAP FLOW IN MAPLE 



437 



of the solute into the receiving reservoir would necessarily occur. 

 The exuded sap, in such cases, might be pure water. 



That the production of comparatively great pressure is possible 

 in either of these ways is apparent. As show^n by Pfeffer's table 

 (p. 146) a difference of one percent in concentration of sugar 

 solution is equal to a pressure of about 0.69 atmospheres or 10.3 

 lbs. (4.6 kg.). Twenty pounds pressure, which is about the 

 maximum for the maple, would be equivalent to a difference of 

 2 ^ in concentration. This is not too great to expect considering 

 that the percent of sugar in maple sap is from 1 to 5.5 and that 

 local concentration might be much greater for a short time before 

 diffusion. One might reasonably expect a higher pressure. 



It is very difficult to bring forward any general theoretical 

 evidence to establish the impossibility of either the excretion of 

 solute theory, or the unequal distribution theory. But the fact 

 that sugar actually passes into the vessels in large quantities lends 

 a probability almost convincing to the idea that the increased 

 permeability allowing the sugar to escape is also the cause of the 

 pressure. 



Maple wood is diffuse-porous, the vessels being scattered 

 rather evenly throughout the annual ring, although they are per- 

 haps slightly more numerous in the spriiii>- wood. The vessels 

 are large, soHtary, or, more usually, two or tlirec loueiher and 

 surrounded by the moderately thick-walled wood filieis. These 

 latter form the main bulk of the woody portion. \\o..d j-aren- 

 chyma is very scarce, and is confined to a few ro\v> of eell> in the 

 vicinity of the vessels at points wliere they are adjacent to the 

 pith rkys. There is some question whether this tissue is woo<l 

 parenchyma since there are no cross walls as ordinarily; or whether 

 the cells are not wood fibers like the rest but with cellulose walls. 

 I am inclined toward the latter view since the similarity is other- 

 wise so striking. The wood fibers are without markings but the 

 vessels are densely pitted. Pith rays are numerous in maple and 

 very large (Fig. 1). An estimate seems to show that they occupy 

 about one fourth of the volume of the wood. The larger ones are 

 from 8 to 10 cells high and from 3 to 4 cells thick, ellipsoidal in 

 tangential section, and extend from the cortex to varying depths 

 into the wood, some reaching to the center. The cells of the pith 



