The vascular system of plants consists of the xylem and phloem. They are somewhat like blood vessels in animals, but plants transport materials using two tissues rather than one. Here is a look at what xylem and phloem are, what they transport, and how they work.
What are Xylem and Phloem?
Xylem and phloem are the two types of transport tissue found in vascular plants. They form a complex network running throughout the plant, carrying resources to different parts and disposing of waste products.
- Xylem primarily transports water and mineral nutrients from the roots to the rest of the plant, and it also plays a role in physical support.
- Phloem transports organic substances, such as sugars produced during photosynthesis, from the leaves to other parts of the plant.
| Xylem | Phloem | |
|---|---|---|
| Functions | Transports water and minerals in support of photosynthesis and transpiration. Xylem also functions as structural support for the plant. | Transports organic molecules, such as sugars, amino acids, some plant hormones, and mRNA. |
| Description | Consists of tubular tissues that lack cross walls and resemble a star shape. | Consists of elongated tubular tissues that have walls with thin sieve tube. |
| Location | Occurs in roots, stems, and leaves, in the center of the vascular bundle. | Occurs in stems and leaves, and eventually roots and fruits, on the outer part of the vascular bundle. |
| Sap Movement | Fluid only moves upward from the roots toward the stems and leaves. | Fluid movement is bidirectional, moving up or down depending on the plant’s needs. |
| Mode of Transport | Negative pressure powers the upward flow of fluid. | Turgor pressure from osmosis powers the flow of sap. |
| Tissues | Consists of tracheids, vessel elements, xylem fibers, xylem parenchyma, and xylem schlerenchyma. | Consists of sieve tubes, companion cells, bast fibers, phloen fibers, intermediary cells, and phloem parenchyma. |
| Features | Mature xylem is dead tissue. The presence of lignin fibers makes xylem waterproof and keeps it from collapsing under pressure. | Phloem is living tissue although the sieve tube cells lack nuclei and contain little cytoplasm. |
Importance of the Vascular System in Plants
The vascular system allows plants to grow taller and larger, enabling them to inhabit a wide variety of environments. Without these conduits, plants only grow to a small size. Non-vascular plants, such as mosses and liverworts, lack xylem and phloem and rely on diffusion and osmosis for the distribution of nutrients. Vascular plants, including trees, flowering plants, and ferns, use xylem and phloem to efficiently transport nutrients, even against gravity.
Xylem
The term “xylem” comes from the Greek word “xylon,” which means “wood.” This reflects the role of xylem tissue in contributing to the structural strength of plants, particularly woody ones.
Function and Structure of Xylem
Xylem transports water and dissolved minerals absorbed from the soil by the roots to the above-ground parts of the plant. The plant uses the water transported by the xylem photosynthesis and transpiration. Additionally, the xylem also provides structural support to the plant.
The xylem tissue consists of four main types of cells: tracheids, vessel elements, xylem parenchyma, and xylem fibers. The vessel elements and tracheids are the water-conducting cells. Vessel elements are wider and shorter than tracheids and connect together at the ends. The ends have perforation plates that permit water transfer between cells. Tracheids are long, thin, and tapered at the ends. The secondary cell walls of the tracheids contain lignin. The parenchyma stores food and helps in the repair and growth of xylem, while xylem fibers provide support.
In most plants, the xylem is in the center of the stem, forming a core of rigid, woody material. Mature xylem consists of dead vessel element and tracheid cells connected by hollow ends.
Transportation in Xylem
The mechanism of water transport in xylem primarily involves a process known as cohesion-tension theory. Here, the evaporative pull of transpiration from the leaves creates a tension or negative pressure that pulls water upward from the roots through the xylem tissue. Also, root pressure also plays a role. Here, water enters roots from the soil via osmosis, generating a positive pressure that forces water upward into the plant.
Phloem
The term “phloem” comes from the Greek word “phloios,” meaning “bark.” This name is fitting, as phloem is often found just beneath the bark in trees.
Function and Structure of Phloem
Phloem transports organic nutrients, particularly sugars synthesized during photosynthesis, from the leaves to all other cells of the plant, including the roots.
Phloem tissue is composed of sieve-tube elements, companion cells, phloem fibers, and phloem parenchyma. The sieve-tube elements, along with their companion cells, primarily control the transportation of food. Phloem fibers provide support, and phloem parenchyma assists with food storage and the secretion of plant resins.
In most plants, the phloem is towards the exterior of the plant, just below the bark in stems and roots. The sieve-tube cells are alive, but they lack a nucleus and have less cytoplasm than other plant cells. The companion cells are living cells with a normal composition.
Transportation in Phloem
The transport mechanism in phloem is known as translocation. It involves an active process where sugars load into sieve tubes in the leaves (source) and unload where they are needed (sink), such as roots or developing shoots. This differential in sugar concentration results in water moving from xylem to phloem, building a pressure that drives the sap down the plant.
Differences in Xylem and Phloem in Monocots and Dicots
Monocots and dicots differ in the arrangement and structure of their xylem and phloem.
In dicot plants, the vascular system is organized in a ring, with the xylem typically inside, surrounded by phloem. There is often a region of meristematic cambium cells, which divide to produce more xylem or phloem cells, allowing the stem or root to increase in diameter.
In monocot plants, the xylem and phloem are paired into bundles scattered throughout the stem. Monocots do not have a vascular cambium, meaning they typically do not increase in diameter after growth.
Girdling and Its Impacts
Girdling is a practice that removes a ring of bark (the phloem layer) from around the entire circumference of a tree or plant stem. This disrupts the downward transportation of sugars and other metabolites from the leaves through the phloem. Girdling can cause the death of a tree because it interrupts the supply of food from leaves to the roots, essentially starving the plant.
However, girdling also has a deliberate use in horticulture. It encourages the plant to produce larger fruits or to direct the plant’s energy towards certain branches. By disrupting the flow of nutrients, the plant overcompensates in the remaining portions, often leading to increased yield or size of the produce.
References
- Lucas, William; et al. (2013). “”The Plant Vascular System ” Evolution, Development and Functions”. Journal of Integrative Plant Biology. 55 (4): 294–388. doi:10.1111/jipb.12041
- McCulloh, Katherine A.; John S. Sperry; Frederick R. Adler (2003). “Water transport in plants obeys Murray’s law”. Nature. 421 (6926): 939–942. doi:10.1038/nature01444
- Raven, Peter A.; Evert, Ray F.; Eichhorn, Susan E. (1999). Biology of Plants. W.H. Freeman and Company. ISBN 978-1-57259-611-5.
- Roberts, Keith (ed.) (2007). Handbook of Plant Science. Vol. 1 (Illustrated ed.). John Wiley & Sons. ISBN 9780470057230.
- Slewinski, Thomas L.; Zhang, Cankui; Turgeon, Robert (2013-07-05). “Structural and functional heterogeneity in phloem loading and transport”. Frontiers in Plant Science. 4: 244. doi:10.3389/fpls.2013.00244