Plants, just like all other living beings, need a circulatory system. The transport system in any plant is much alike from a three thousand year old giant sequoia to the tiniest weed hidden deeply within the forest or meadow. Nutrients must transport to both leaves and roots. Also, plants need water and minerals. These, too, are carried by the circulatory system within the structure of the plant.
The plant can be thought of as a vertical elevator system. The elevators' "cars" are carried up in a curious one way direction by the elevator “shaft” which is called the xylem. The xylem is the delivery system to shoots, stems and leaves. It contains the vascular system of the plant. Think of a one-way elevator moving up from the roots of the plant, delivering nutrients and minerals.
The other elevator car moves through a shaft that goes both up and down. This part of any plant is called the phloem. The phloem consists of tubes, or in this metaphor, shafts and cell tissues that both store and move water and nutrients up or down as needed. An important function of the phloem is to extend the roots, providing surface for all needed nutrients to be collected and stored. Roots have their own jobs in the elevator metaphor of any plant. They anchor the plant in soil. They absorb minerals and water to supply the vascular system. They also store these vital nutrients.
The column of the plant, or trunk, keeps the xylem and phloem separate by a layer called the cambium. Cambium is divided, with the inner portion of it containing the xylem and the outer portion of it connecting the phloem. The area surrounding the cambium is called the cortex. The innermost center is called the pith. Finally, the outermost shell, also known as bark, is called the epidermis. This is the same term used for skin in all animals, of course, including humans.
All plants are miracles of evolution in the myriad functions they perform. They use light from the sun, which is gathered by chloroplasts to create the food and minerals the plant needs. This is called photosynthesis, which drives the carbon/oxygen exchange cycle. Transpiration is the process used to move water and minerals gathered by leaves and roots to also create life-giving oxygen. This exchange of oxygen and carbon dioxide (CO2) that plants “breathe” out is what makes all life possible. All plant and marine phytoplankton are the lungs of the earth. They are therefore collectively, in a sense, the transport system of nutrients, minerals and water for the whole biosphere.