Throughout a flowering plant are tubular cell structures that provide for the movement of both food and water. Water is transferred through structures called xylem. Xylem is made up of dead, empty cells aligned end to end with an opening at the ends of each cell. The tubes are microscopic in diameter, and there are many of them arranged in bundles. The simple sugars manufactured in the leaves are transferred throughout the plant in another tubular system called phloem. Unlike xylem, phloem is made up of two kinds of cells, both living.
Roots, stems and leaves all contain xylem and phloem arranged together in bundles surrounded by another sheath of cells. These bundles of tubes are the same regardless of where they are in the plant. Xylem and phloem in the roots are no different than in the leaves or stems. The veins visible in plant leaves are, in fact, bundles of xylem of phloem.
Roots and Rootlets
The roots of a flowering plant have tiny hair-like structures called rootlets. The rootlets are the primary means of taking in water. The process happens through osmosis. The outer cell walls of the rootlets are porous membranes with openings that allow water and minerals to pass through.
Pressure in the soil, caused mainly by the weight of the soil, causes water to be squeezed through the outer membranes of the rootlets. Once in the rootlets, water is transferred through the plant by another process called transpiration. Note that plants only use about 1 percent (on average) of the water they take in for photosynthesis. The rest of the water is lost through the transpiration process, which moves water up from the roots, through the stems and into the leaves.
Transpiration relies on the physics of the molecular cohesion of water. Molecular cohesion happens because water is a bipolar molecule---two oxygen molecules bind with a single hydrogen molecule. The molecules are attracted to each other and make long chains of molecules. The oxygen atom of one water molecule is attracted to the oxygen atom of another water molecule. The molecular cohesion bonds are so strong that within the xylem of the plant, the water molecule chains are nearly as strong as steel wires of the same diameter.
The long chains of water molecules begin in the roots and remain unbroken in the xylem of the roots, stems and leaves. In the leaves, water molecules evaporate through pores. As each molecule evaporates in the air, it pulls the molecule behind it up, along with the rest of the chain of water molecules. This transpiration process is how flowers, and other higher plants, drink water. In a similar fashion, as chlorophyll uses water molecules to make food, each molecule used pulls the chain of molecules behind it up.