30 EXPERIMENT STATION BULLETIN 354 



There is some evidence that the quantity of sap is directly influenced 

 by air temperature. To determine this relationship a series of thermo- 

 couples were installed at several places inside the bark of red maple trees. 

 Reel maples were used because there was not sufficient num])er of sugar 

 maples of a satisfactory size growing close together. This part of the ex- 

 periment led to some very interesting and important results which are be- 

 ing prepared for publication. Trees about 8 inches in diameter and 40 feet 

 tall were severed from the stump and set upright in barrels of water. One 

 was left with all branches intact, one was stripped of all branches, and one 

 was stripped of all l)ranches and placed in the barrel upside down. Each 

 was tapped and a spout inserted. Sap of normal cjuality flowed from each 

 tree on the same days and during the same hours that it flowed from con- 

 trol trees nearby. In the same way, sap flowed from each section of a tree 

 cut into eight-foot logs and set into a barrel, whether upright or inverted. 

 No sap flowed from spouts in two eight-foot sections of the same tree which 

 were placed upright on a dry base. One section was placed horizontally 

 and the butt end inserted in an old inner-tube filled with water. Sap flowed 

 from each of three tap holes spaced at intervals of two feet. It was later 

 cut into pieces two feet long and these were set into a bucket of water and 

 tapped. A tiny quantity of sap was produced, but this was so late in the 

 season that the experiment was not considered satisfactory. In each tree 

 and in each section all cut surfaces not immersed in water were sealed 

 with grafting wax to prevent undue drying out of the tissues. 



The results of this experiment lead to the conclusions that root pres- 

 sure, so-called, is not necessary for the flow of maple sap in early spring. 

 Nor is there needed any lifting power which might be ascril)ed to transpi- 

 ration in the buds, twigs, or branches. Observations of the internal tem- 

 peratures of the tree and of the fluctuations of the water in the barrels 

 around the base of the severed trees indicate that sap flow is influenced 

 largely by the temperature of the air. 



It has long been a maxim that maple sap flows best on a warm day 

 following a freezing night. This was observed in all our experimental 

 trees. The thermograph inside the tree indicated that it takes some time 

 for external temperatures to penetrate to the heart of the tree, and that the 

 wood at the center is still relatively warm after the temperature of the air 

 has dropped several degrees below freezing. The thermocouples in the 

 cambium region, however, showed that this area cools ofif rather rapidly, 

 and the sap here freezes long before that deeper in the wood. This is es- 

 pecially noticeable in the upper parts of the tree. Ice crystals form in the 

 tissues here, resulting in a more concentrated cell solution, due to the su- 

 gar present in the sap. This sets up a tension which draws the sap from the 

 interior of the tree toward the exterior, where it freezes. As the tem- 

 perature drops, this freezing penetrates toward the heart and down the 

 trunk, and more and more moisture is drawn up from the roots and the 

 lower part of the trunk. When the temperatures goes low enough the 

 whole trunk is frozen and no more moisture is absorbed. On the follow- 

 ing day, as the air warms, the ice melts and the sap flows out at any con- 

 venient opening. 



As will readily be seen, the longer it takes for the interior of the tree 

 to freeze the more sap will be drawn up into the tissues, until the satura- 

 tion point is reached. If the temperature drops rapidly and goes quite low, 



