From a botany site on syrup:
"Most people associate plant sugar with phloem and assume that sugar maple sap comes from the phloem. Not so! Sugar here comes from the wood, and the spout is driven into the outermost sapwood. In late summer and before it loses its colorful leaves in the fall, this tree stores large quantities of starch in the wood parenchyma. Then when temperatures rise in late winter, the starch is broken down and converted into sucrose, which is released into the wood vessels. The high concentration of sugar in the vessels causes soil water to be brought (diffuse) into the roots, building up pressure in the root and forcing the sugary sap upwards toward the unopened, dormant buds. Alternating freezing or cold nights and warm days causes sharp changes in xylem water pressures; flow is greatest in the warm part of the day and stops at night. Seasonal flow lasts for two to six weeks, and it stops when buds begin to open and when all of the initial liquid with the sucrose has ascended the plant. The amount of sap removed from a tree, typically about one gallon, is not detrimental to the growth of the tree"