Xylem: A plant tissue consisting of various types of cells, which transports water and dissolved substances towards the leaves. It can be distinguished from the phloem by the presence of vertical systems of dead cells with thick, lignified walls.*
The sweet sap of Acer saccharum (sugar maple) flows through the xylem in its trunk. Spring is the only time the sap in the xylem is so full of sucrose. Later in the year, the xylem will only be transporting water from the roots to the leaves.
*Allaby, M. (2006). A dictionary of plant sciences (Revised ed.). Oxford: Oxford University Press.