No. 474] 



SAP FLOW IN MAPLE 



423 



€ase than it could pass out itself. However, the resistance in any 

 case would be so great that to conceive of even a fourth of the total 

 expansion being transmitted to the single small tap-hole is very 

 difficult. Then, too, if the fibers are nearly filled with gas as 

 seems true in many cases, early in the season at least, the expansion 

 could be but slight l)efore the limits of the cell cavity would be 

 reached. As there would be no more sap to be forced from the 

 cell and as the passage of the gas is difficult, the pressure at the 

 tap-hole would necessarily cease altogether. 



The gas-expansion theory cannot account for the {)ressures 

 obtained, and can account for the volume of flow only by assuming 

 very improbable conditions. It seems to be really out of the 

 question. Sachs' interj)retati()ns were doubtless correct for the 

 phenomena investigated, but the conditions in the maple tree are 

 not of the same nature as those in the blocks of wood used in his 

 •experiments. 



The Water-expaxsiox Theory 



One of the earliest as well as one of the most general of the 



t\w ."X|,;in.i..n ..r \hv iiM'lt';.. tlu" iciiip.Tntiirc n.^c on a good 



j)ose that (he waicr can l.c forced fn.in ihe vosrl^ iiit<. the wood 

 fibers oiiK with M.inc <lifiiciili\ . >a<'h. dioue.l that n.ii.idcrable 



direction, and the \'(M-iii()]it (^xperiiiuMits show that httle ])ressui-e 



is transmiTt<Ml huerally. Therefrnv if the temperature ri.e. (,uiekh 

 in the mornin- the e\paii<hnu u aier ur^^ l.e pre\ eiiied fn.m finu inn 



time. When later the u'l'er had penetrated the w....d libers, this 

 pressure would rapidly tall jn^i a^ it aetually does fall in the maple 



