422 



THE AMERICAN NATURALIST 



[Vol. XL 



Is the gas-expansion theory capable of accounting for the flow 

 from the maple? A sap flow as high as 20 liters a day has been 

 recorded in some instances, but under ordinary conditions the flow 

 rarely exceeds 10 liters. If we take a tree 5 dm. in diameter and 

 20 meters high, and assume that the branches if pressed together 

 in an erect position would approximately complete a cyhnder 

 w^ith a diameter of the lower trunk and a height of the tree, we find 

 that the volume of the tree would be approximately 3.927 cu. m. 

 As given by the Vermont Bulletin, about 25^0 of this volume is 

 gas or about 981,745 cc. This would expand for every degree 

 3596.1 cc. For a rise of from 5 to 10 degrees the expansion would 

 be from 17,980.5 cc. to 35,961.0 cc. If we were justified in assum- 

 ing the transmission of pressure without friction from all parts 

 of the tree then this gas expansion would easily cover the ordinary 

 10,000 cc. flow, and even the maximum of 20,000 cc. But we 

 are not justified in such an assumption. Resistance within the 

 tree is great, and pressure, according to the Vermont studies, is 

 not transmitted more than eight feet either way from the tap-hole. 

 Therefore in such a section of the above trunk 16 ft. long (487.5 cm.) 

 and hence containing 239,300 cc. of gas, a rise of one degree would 

 cause an expansion of 876.5 cc. or 8765 cc. for 10 degrees. This 

 would barely account for the ordinary flow of 10,000 cc. It is 

 not true, however, that the whole of the water, even in such a. 

 section of the trunk, is in frictionless connection wnth the tap-hole. 

 Water travels with great diflSculty transversely from one annual 

 ring to another, so that the outer layers only would probably fur- 

 nish the main quantity of the sap. In such case the gas expansion 

 of these layers would probably be only from i to | of the whole 

 amount for the above section of trunk, and would scarcely account 

 for even the smaller daily flows. 



Moreover, the gas, as shown above, is at this season mostly, 

 if not entirely, confined in the wood fibers. Gases difi'use through 

 moistened cell walls only with difficulty so that a heaw pressure 

 and considerable time would be required before the diffusion could 

 be of much magnitude. In answer to this objection it might be 

 claimed that if the wood fiber, instead of containing air alone, 

 were partly filled with water, as is frequently true, then the ex- 

 pansion of the gas might press the liquid out with much greater 



