Your essay should answer the following: describe the cross-section of the thin celery slice. what does it look like? try to identify the "stringy" transport bundles of xylem and phloem. what color are they? how does this prove they carry water and minerals up the stem? look carefully at the leaves of the celery. what color are they? explain the change in color of the leaves that you saw.

1) The stringy transport of xylem and phloem becomes the color of whatever the food coloring color is. 

2) This proves they carry water and minerals up the stem, because if the food coloring does not effect the rest of the stem, it will stay in that same place.

3) the color of the leaves are whatever the food coloring 'color' is. For example, the celery that is placed in red-food coloring has reddish leaves
The Celery placed in blue has bluish-leaves, etc

hope this helps