...is for Xylem...
...the tubes in a tree which transport water from the roots to the leaves. The water travels by two methods, transpirational pull through negative pressure that pulls the water from the roots and soil or by root pressure, water traveling by osmosis (a difference in pressure) into the root from the soil creating a positive pressure that forces liquid up the xylem. As the water evaporates from the leaves more is drawn up through the tree to replace it. These tubes only live for one year, new xylem are grown each year from the cambium, a layer of tissue between the inner bark (phloem) and the xylem. As the tree grows the inner rings of xylem become the heartwood, the outer rings the sapwood.
The dead xylem is visible as the rings of a tree when it is cut down.
Joining in with the Alphabet Photography Project