A student holds a leaf up to the light and sees a fine network of veins running through it. How are xylem and phloem arranged within that network?

Explication

The two transport tissues run together in bundles, so one vein gives a leaf both a route in and a route out. Treating them as two separate systems placed in different organs cannot work, because a single leaf has to receive water and export sugar at the same time and both routes must reach the same place.