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Plants rely on their vascular system to transport water and nutrients, with xylem transporting water and phloem moving organic nutrients. The system is found in vascular bundles and includes a layer called the vascular cambium, which segregates xylem and phloem and is responsible for stem cell differentiation.
The plant vascular system, filled with tube-like vascular tissue, are the crucial conduits through which water and nutrients flow in plants. Without them, plants are limited to a few inches (centimeters) tall, as is the case with mosses, hornworts, and liverworts, the only non-vascular land plants. Most land plants alive today have a vascular system, from small shrubs to the tallest trees. The largest carries water and nutrients to the tops of trees over 379m tall, such as the giant sequoia.
The vascular system in plants uses more than one type of cell. Tissues are divided into two parts: xylem and phloem. Xylem is found closest to the center of the plant and its main function is to transport water. The best known xylem is wood. It is composed mostly of dead cells with a tubular structure. The phloem, however, consists mostly of living cells. Located closest to the outside of the plant and just below the bark of trees, the phloem moves organic nutrients (known as photosynthetics) to the top of the plant. The plant’s dependence on phloem is why trees can be killed simply by removing a circular layer of bark around their circumference.
In plants, this system is found in coordinated blocks called vascular bundles, which contain xylem, phloem, and protective cells around their perimeter. As mentioned, the xylem is found closer to the center, while the phloem is closer to the edge. In leaves, the xylem is at the top, the phloem at the bottom. For this reason, aphids are often found on the underside of leaves: this is where they can penetrate to obtain sucrose from the plant juices.
Between the xylem and phloem is a layer known as the vascular cambium. It segregates xylem and phloem from each other and is the source of the stem cells that differentiate into both. The growth rate of vascular cambium cells depends on available sunlight and nutrients, and rings found in tree trunks are indicative of vascular cambium growth patterns.
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