There are two types of tissues found in vascular plants that are used for the transport of water, minerals, sugar, nutrients, and amino acids. These tissues are known as xylem and phloem. While they serve somewhat similar functions, there are a number of significant differences between the two. The following is a closer examination of these two tissue types, their similarities, and their differences.
Xylem comes from the Greek word xylon, which translates to “wood.” It earned this name in the 19th century from Carl Nageli because wood is the most well-known xylem tissue. Though it may be most prominent in the wood tissue, xylem is still located throughout the rest of the plant.
The main function of xylem in most vascular plants is the upward transport of water and minerals from the root systems. Any substance transported by xylem has unidirectional movement meaning that it can only travel in one direction: upwards. Because of this, the xylem does not carry any substances back to the roots.
The mixture of water and minerals transported by the xylem is referred to as xylem sap. Transport of the sap upwards occurs passively and requires no energy expenditure from the plant. This makes it more difficult for the xylem sap to be properly transported as a plant grows taller, As a result, the maximum length that the sap can flow will often limit the height of the plant, which is especially evident in trees.
The three passive functions that cause sap to flow upward through the xylem are root pressure, transpiration pull, and pressure flow. Root flow occurs when water moves into the roots from the soil by means of osmosis. As this occurs, pressure accumulates in the root system and begins to force the sap upward through the xylem. Additionally, it’s possible for the pressure to become so great that the sap is eventually pushed out of the leaf through guttation.1
The pressure flow method of movement directly involves the relationship between xylem and phloem. As there are often intersections between the two travel systems, an increase in pressure in the phloem can cause negative pressure in the xylem that pulls the sap upward. This combines with the transpiration pull method, which occurs when water evaporates from the surface of cells to create negative pressure that pulls the sap through the xylem.
One additional theory to explain the movement of water and minerals in the xylem is the widely-debated cohesion-tension method. This theory suggests that the attractive force between water molecules allows water to move upward through the xylem. As one water molecule is eventually lost, another is pulled up and the sap continues to flow.
Phloem is the second form of tissue used for transport in vascular plants. It gets its name from the Greek word phloios, which means “bark.” Carl Nageli also coined this name in 1858 because phloem is found in the innermost layer of bark.
The role of phloem is to transport food and nutrients produced during photosynthesis throughout the plant. Unlike the sap moving through the xylem, the transportation of these substances occurs via bidirectional movement through the phloem tissue, meaning it can move either up or down through the plant.
The movement of food and nutrients via the phloem is referred to as translocation, and it is said that translocation of sugar happens between a “source” and a “sink.” In the case of sugars produced during photosynthesis, the leaves are the source and the sinks include roots, bulbs, and ground tissue.2
Sieve elements are special cell types used to transport the sugar through the phloem. In most cases, the sieve elements translocate the sugar from the source to the nearest sink. Leaves near the top of the plant translocate sugar up to the tip of the plant while lower leaves translocate sugar to the roots.
Furthermore, the phloem of a plant can be manipulated via a process known as girdling. This involves removing a strip of bark from a plant in a ring shape and prevents the flow of nutrients past the point of the ring. If this were to happen to a tree, it is likely a tree would die.
However, an individual branch can be girdled to keep all of the nutrients produced on that branch from moving. If all but one fruit were then removed from the branch, the single fruit would be the only “sink” and would thus absorb all of the food and nutrients the branch produced. This technique is often used in agriculture to produce abnormally large fruits and vegetables.
Differences and Interactions Between Xylem and Phloem
One major difference between xylem and phloem is that most cells in the xylem are dead while the phloem is composed of cells that are still alive. In the case of both structures, it is changing in pressure that results in the flow of sap. The difference, however, is that negative pressure pulls xylem sap while positive hydrostatic pressure forces sap through the phloem.
When these two tissues are found together, they are referred to as a vascular bundle. These bundles are common and tend to stretch the length of most stems and roots. The xylem is often located on the interior and the phloem on the exterior in the stems of eudicots.
In roots, the bundle grows within the stele, or central section, of the root. In a eudicot, the xylem grows n a cross-pattern of cells with multiple phloem systems growing between each section of cross-pattern. In a monocot, the two form a circular pattern. In both cases, the vascular bundles from the roots system eventually merge with the bundles from the stem at the base of the plant.
Understanding these two tissue systems is an important part of understanding how plants function and grow. Both are necessary for the survival of vascular plants and both can be manipulated to produce unique agricultural results.
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