It is Xylem of the plants that help transport the water and other nutrients from the roots up through the plant. The phloem basically transport food synthesised by the plant but not the water.
The generalization that all organisms are composed of cells was established through multiple observations in various kinds of animals and plants. A controlled experiment of finding cells in all the kinds of living organisms is not possible or is infeasible as it will require prior knowledge of all the kinds of organisms inhabiting Earth and the effort to catch and bring them in the controlled environment of the lab for testing.
Natural selection is the idea that species that acquire adaptations favorable for their environment will pass those adaptations to their offspring. Eventually, only individuals with those favorable adaptations will survive.
