Phloem and xylem are plant tissues that transport different substances to and from different parts of the plant. Phloem transports dissolved substances, while xylem transports water. Unlike mammals, plant cells don’t need substances as quickly, and the transport system is different. Phloem and xylem have different transport tasks and are arranged as long tubes throughout the plant. Phloem tissue is made up of sieve tube elements and companion cells, which perform living functions for both themselves and their sieve tube element.
Phloem is a type of tissue found in plants. Together with the xylem, the phloem forms the transport system within plants. Phloem and xylem are completely separated within the transport system. Both tissues are found in all plants and each transport different substances to and from different parts of the plant. Phloem is used to transport dissolved substances, sap, around the plant, while xylem transports water.
The transport system of plants differs from that of mammals in two distinct ways. Plant cells don’t need substances as quickly as mammals because they are much less active. Water and dissolved substances move through the plant by diffusion and osmosis, they are not driven by a pump, like the heart. The other difference is in what is transported in the tissues of the transport system and where.
In mammals, blood is transported through the transport system with arteries taking blood away from the heart and veins bringing it back. In plants, substances and water move in all directions, up and down and to and from the leaves. Furthermore, phloem and xylem have completely different transport tasks, one transports water and the other transports food. Through the phloem tissue, the sugars produced in the leaves during photosynthesis are transported to all other parts of the plants to be used in cellular functions, growth or to be stored in the roots.
The phloem and xylem tissues are arranged so that the cells join end to end to form long tubes throughout the plant. The cells that make up the conveyor system have been modified to allow for easier transportation through the facility. In xylem tissue, the cells are dead. The terminal cell walls are completely broken down to form a hollow tube which allows for the rapid transport of water and also provides support to the plant. The phloem cells are not dead and only play a transport role; they are also not used as a stand.
Phloem tissue is made up of two different types of cells, each with very specific roles. The actual tube in which the sap flows is called the sieve tube and consists of sieve tube elements. The cell walls, or sieves, of these cells are not completely broken down as in the xylem, but have small pores or holes. The pores allow dissolved nutrients to move easily through the tube. Unlike most living cells, sieve tube elements do not have a nucleus and have only a very thin layer of cytoplasm with very few organelles.
The other cells that make up the phloem tissue are called companion cells. Due to the lack of a nucleus and organelles in the sieve tube elements, it would be very difficult for them to live on their own. For each element of the sieve tube, there is a corresponding companion cell which contains a nucleus, organelles and a very dense cytoplasm filled with nutrients. Companion cells perform living functions for both themselves and their sieve tube element. These two cells are actually formed from a single cell during the development of phloem tissue.
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