How water moves up
The single most important thing about xylem is that the cross walls between its cells are gone.
The xylem vessel
Xylem vessel cells join end to end, and the cross walls between them dissolve away, so that the whole row becomes one continuous tube-like structure — the xylem vessel. This tube contributes to the transport of water vertically through the plant.
Tracheids
Tracheids are long, spindle-shaped cells, tapering to a point at each end. They also contribute to water transport. Unlike the vessel cells, tracheids keep their end walls, so water passes between them rather than straight through — slower, but a tracheid is less likely to let an air bubble spread the length of the plant.
A tube with no floors in it
Picture a stack of drinking glasses with the bottoms knocked out. That is a xylem vessel: many cells, one continuous channel, running from the root to the topmost leaf without a single wall across it. The cells had to die for this to be possible — a living cell cannot have a hole in the middle of it.
Where the water comes in
In grade 10 you met the root hair cell, long and narrow so as to give more surface for absorbing water. This is where that water goes. It is taken up by the root hairs, passes across the root, enters the xylem, and travels up the vessels to every part of the plant.
This is the first time transport in plants has been explained
Until now you knew that roots absorb water and that leaves make food, with nothing joining the two. The xylem vessel is that join: a pipe made of dead cells, running the whole length of the plant. The next skill supplies the pipe that carries the food back the other way.