Vascular plants are plants with specialized tissue that act as a pipeline for carrying the food and water they need. All plants, except bryophytes like mosses, liverworts, and hornworts, have developed special, internal systems to transport their requirements from one part to another. Vascular plants have systems that move nutrients and water from the soil to the stems and leaves and transport food to where it is needed.
A green plant is about 90 percent water, but it loses much of its water through evaporation from its leaves. Because of this, plants need a steady supply of water to grow and reproduce. Much of this need for water is satisfied by the plant's root system that develops in a network spreading down and out from the stem. Once the roots reach water, however, they must be able to get the water back up to the branches and leaves. Plants are able to achieve this by having tissues, or cells that are specialized, to perform a particular function. Tissue moves water is called vascular tissue. Vascular comes from the Latin for "little vessel." There are two types of vascular tissue in a plant: xylem tissue and phloem tissue. The xylem is a ring or a series of hollow tubes that extend from the roots, up through the stem, and out to every branch and leaf. The xylem is located near the surface of the stem and in a tree, it forms a band just below the bark. The xylem is made up of dead, hollow cells arranged end-to-end so that they look like a tube or a straw. The xylem carries water and minerals, like sap, from the roots on a one-way trip to the stem and leaves.
The second part of a plant's circulatory system is the vascular tissue called phloem. Unlike xylem, the cells that make up phloem are living. The phloem carries food (in the form of organic molecules) that the leaves and stems have made by photosynthesis (the process by which plants use light energy to make food from simple chemicals) to parts of the plant that are unable to make their own food (such as the roots and stem tip). In a vascular plant, xylem and phloem are always found together in groups that are called vascular bundles. These bundles may be distributed evenly through the body of a stem, grouped in rings near the outside, or found in a ring in the outer part of the stem according to the type of plant.
Water and minerals are moved from the belowground roots to the aboveground stem and leaves by a phenomenon known as water potential. Although plants do not have a pump (as animals have a heart) to push water up from their roots, they nonetheless are able to move water upwards by water potential without expending any energy. Water potential works according to the laws of physics. When a wet and dry place are joined by a tube of water, the water always flows toward the dry area. This is similar to the law of physics that says when hot and cold objects come together, the heat always travels toward the cold. In a plant, there is usually a large moisture difference between roots (which are surrounded by water) and leaves (which are constantly losing water through their surface pores). This steady water loss is called transpiration. As the roots absorb water, the pressure builds up in them, causing the water to move up the xylem to the drier parts of the plant (stem and leaves). As a result of water potential and transpiration, water always moves on its own from the roots to the leaves.
Plants also rely on the water supplied by their vascular systems to keep their shape. Water that is drawn into its cells create what is called "turgor pressure" and, like an inflated balloon, the cell becomes full or rigid. If a plant loses more water through its leaves than it is able to take in from its roots, it will lose some of this turgor and begin to go limp or wilt. If such a situation continues for any length of time, the plant will eventually die.
[See alsoPlant Anatomy ]