Inside every plant stem there are hundreds of tiny tubes called xylem vessels — too small to see without a microscope, but your coloured water just made them visible. 

These tubes run continuously from the roots, up through the stem, and out to every leaf. 

When you split the celery stalk in two, each half is connected to different xylem tubes, which is why each colour stayed separated all the way up.

But here's the question that trips everyone up: how does the water get there? Gravity is pulling it down, and a suction pump — like sucking through a drinking straw — only works to about 10 metres. 

The tallest trees reach over 100 metres. So something else is doing the work.

The answer is cohesion-tension. Water molecules cling to each other like people holding hands in a chain. As leaves lose water vapour through tiny pores (transpiration), they pull on the water in the xylem below — and that pull travels all the way down the chain from leaf to stem to root. The water isn't pushed up from below. It's pulled from above.

The coloured dots you saw when you cut the stem? Those are the xylem vessels — each tiny dot was carrying coloured water all the way from the glass to the tips of the leaves.