We filled a glass with water and added a lot of liquid food coloring. We put the celery in the glass and waited over night.
By the next morning, we had evidence that the colored water had moved all the way to the top of the celery stalk through osmosis. The leaves were splotched with blue.
When we cut a cross-section, the main channels were also distinctly blue.
It was even more dramatic to pull the celery apart and see those same channels along their length.
We also noted that the celery was much stiffer, and crisper, and could hold its shape better. I explained that this was why we had a high-humidity drawer in the refrigerator, why the supermarket uses misters in the produce area, why there is water in pre-cut vegetable packages, and why I sometimes soak my older lettuce in water before making a salad.
There's an explanation here: http://www.biologycorner.com/bio1/diffusion.html
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