No. 474] 



SAP FLOW IN MAPLE 



425 



one very weak point. This is the assumed high impermeabiHty of 

 the fiber walls. Although it is probably impossible to force water 

 through many such walls with the pressure observed, still we are 

 scarcely justified in assuming that one or two walls only separating 

 the vessels from the adjacent fibers would be so highly impermeable. 

 This seems improbable. 



To determine whether this theory will account for the volume 

 of flow a few computations must be made. In the Vermont 

 Bulletin the trunk of a certain tree was computed to contain 1220.5 

 lbs. (553,609.6 cc.) of water (p. 62). The coefficient of expansion 

 of water between 4° and 8° C. is 0.000,118 for the whole 4 degrees, 

 which would give an increase in volume of 65.3 cc. for the whole 

 amount. A rise from 8° to 15° C. with a coefficient of 0.000,729 

 would give an additional 403.5 cc. Since tree temperatures vary 

 only a few degrees the expansion in any case would be only a very 

 small fraction of the whole flow. For a tree 20 meters high and 

 5 dm. in diameter, the volume would be 3.927 cu. m., provided 

 that the branches if pressed together would approximately fill out 

 the trunk cylinder to the total height of the tree. If a cubic foot 

 of dry maple wood weighs 43.08 lbs. the dry tree would weigh 

 2,709,895 gms., 45fo of the tree is water, and 55^ is wood; there- 

 fore, the weight of the water would be 2,217,332 gms. The ex- 

 pansion from 4° to 8° C. would be 261.7 cc. If it were possible 

 to believe that the water of the whole tree could flow to the tap- 

 hole without resistance, the flow would be still only one fourth 

 to one half of the actual daily flow. Since, however, it has been 

 shown that pressure is transmitted only a!)ont ei<iht feet each way 

 from the tap-hole, the volume of water which expansion might 

 cause to flow to the tap-hole would be only a fraction of tlie whole. 

 If water passes into the wood fibers to any extent the flow due 

 to expansion would be still less, only about to ,] of tlie whole 

 flow. 



The wood of the majority of trees is structurally so constituted 

 as to render the passage of water diflicult radially from one annual 

 ring to another, while at the same time there may be fairly good 

 communication laterally owing to the bordered pits (in the Coni- 

 fers) and the anastomosis of vessels in the broad-leaf trees. It is 

 very probable, therefore, that not the whole 16-foot (4.8 meters) 



