No. 474] 



SAP FLOW IN MAPLE 



431 



into the cavities of the wood fibers and thus compressing the gas 

 there present. This fiUration, ahhough too slow to offset all the 

 pressure early in the day would materially decrease it, and later 

 overcome it altogether. But the gas has meanwhile become 

 warmed and tends to force water back into the vessels. If the 

 tap-hole is open, a flow due to the gas expansion would occur. 

 Such a flow would be greatest early in the day when the wood 

 expansion was also acting and gradually decrease but at a much 

 slower rate than the decrease of pressure, due to the great elasticity 

 of the gas and to the fact that the maximum volume of the gas 

 would be at maximum temperature. The retarding effect of the 

 slow conduction of heat would probably postpone the maximum still 

 later. If we assume, therefore, that the expansion of gas at these 

 temperatures is great enough to account for the volume of flow, 

 then this theory^ might possibly furnish a means of accounting for 

 the pressure and flow together. 



The objections are several, and fatal to the theory. The most 

 important objection lies in the probabilitv that gas expansion 

 cannot account for the flow, as outlined al)ove un<l.>r tlio <Hsciission 

 of that theory. Much conhl tlie pis account for the tl.nv if 



walls of the vessels, as our prvsvut theory .Ic.nancls. A-ain, as 

 described luider the walcr-cxpansion theorv. we have no u'ood 

 reason to assume such an ahno.l al.sohite inipenneahihty of the 

 cell walls to water. A-ain, curves plotted from tables Wl and 

 X\'ll of the Vermont report show that the sai)-ilow and presstn-e 

 enrves an- praenVallv parallel. The niaxinnnn prosnre and niaxi- 

 ninni (lou- are coincident, an.! both decrease o-raduallv and e(|ually 

 as the day advances. The niaxiinniu of each occurs usually 

 within an hour or two after startinu'. and tlie fall begins some time 

 before the maxinn.m temperature of the air, much less of tlie tree. 



This tlieory also nnist be laid aside as improbable. 



