experiment about water transport in xylem

[zeenash]

An experiment is carried out to show that xylem is responsible for the transportation of water in plant.
A plant which soil on roots has been removed is used in th eexperiment. The roots of the plant are bathed in a stained solution, after three hours, thin cross sections on stem, roots and leaves are cut and the sections are examined under microscopr. Red stain substances are found in where xylem is located.
It is claimed that xylem is responsible for the transportation i plants then.
However, I would like to ask, whether experiments which show that the stain is not related to the water uptake through xylem are necessary to be carried out before the conclusion is made?
That is, is it a must to show that the stain is independent from the water uptake through xylem before the conclusion is made?