No. 474] 



UP FLOW IN MAPLE 



429 



below what we should expect. Supposing, however, that it is 

 able to account for the flow, is there any way of combining this 

 with the water expansion so that the water expansion will account 

 for the pressure and the gas expansion for the flow ? 



Early in the sap season the wood fibers appear filled with gas 

 while the vessels are nearly or quite saturated with sap. The 

 sunlight falling upon the tree in the early morning would rather 

 abruptly warm the outer layers several degrees. Provided now 

 that the walls of the fibers are slowly permeable to water, as has 

 already been shown to be the case to some extent, then a high 

 pressure would be produced by the expansion of the water in the 

 vessels. Rapid expansion would quickly diminish, however, and 

 the slow filtration of sap into the wood fibers would at first counter- 

 balance the remaining expansion, then finally reduce the whole 

 pressure gradually to nearly zero. Fall in temperature late in the 

 day would aid this. Since some of the sap has passed into the 

 wood fibers there would be suction at first at night. This would 

 gradually decrease as the night progresses owing to the return 

 filtration from the wood fibers. This theory could therefore 

 account for the fall in pressure before the maximum temperature 

 in the outside air is reached. Since the fall of temperature is 

 always more gradual than the rise, due to the direct rays of the sun, 

 the suction at night would never be as great as the day pressure, 

 and it never is. After several days of rapid fluctuation of temper- 

 ature, or after a period of warm weather, gas might separate in 

 the vessels thereby rapidly diminishing the pressure to be ob- 

 tained by equal fluctuations of temperature. This fact would 

 account for the cessation of flow during a protracted thaw. A 

 continued period of cold, however, might cause the absorjttion of 

 gas and the resaturation of the vessels. 



The flow can be accounted for by this theory only l)y supjmsing 

 the wood fibers but partly filled with gas, thus alk)wing expansion. 

 The expansion of the gas within the cells as the temperature rises 

 will gradually force sap out into the vessels. The sap will then 

 flow from the tree if tapi.e.! until the gas expansion is completed- 



