All living cells require water and nutrients. If an organism is a single cell or if its body is only a few cells thick, water and nutrients are easily moved through the organism by diffusion. However, diffusion is generally too slow for even small plants to meet their water and nutrient needs. In plants, this problem was solved with the evolution of a specialized system for fast and efficient long-distance transport of water and nutrients. This specialized cellular network is the vascular tissue system; plants with vascular tissues are referred to as vascular plants.
The vascular tissue system is composed of two different types of tissues: xylem and phloem. Although both xylem and phloem form a continuous tissue system throughout the plant body, the two tissues have different functions. Xylem is the primary water- and mineral-conducting tissue, and phloem is the primary food-conducting tissue.
Unlike the circulatory system in animals, the vascular tissue in plants does not recirculate water. Instead, water takes a one-way journey from the soil upward through the plant body to be lost to the atmosphere through evaporation. The watery journey occurs within the xylem tissue. In contrast, phloem tissue transports dissolved sugars (food) from regions where sugars are made or stored (sources) to regions where sugars are required for metabolic processes (sinks). Phloem transports sugars from source to sink. Source sites include photosynthetic tissue, usually leaves, where sugars are manufactured, and storage organs (thickened stems or roots, such as the root of a sugar beet).
Freed from the requirement to hug a moist soil surface, plants with vascular tissue can grow tall, extending their complex stems and leaves into the dry air. Vascular tissue, along with several other important plant features, allowed plants to colonize Earth's surface. Today, our planet hosts an enormous diversity of vascular plant life, including such different forms as ferns, redwood trees, oak trees, and orchids.
Vascular tissue develops in all organs—root, stem, and leaf—of the plant body. In the primary plant body, vascular tissue differentiates from a primary meristem, the procambium. Xylem and phloem tissues that differentiate from procambial tissue are called primary xylem and primary phloem. In plants with secondary growth, vascular tissue differentiates from a lateral meristem, the vascular cambium, producing secondary xylem and secondary phloem. Secondary xylem is a familiar product: wood.
Xylem: The Water-Conducting Vascular Tissue
Xylem is a complex tissue composed of several different cell types. This tissue includes parenchyma cells, fibers, and two cell types specializing in water and mineral transport: tracheids and vessel members. Collectively tracheids and vessel members are called tracheary elements.
The tracheid evolved first, appearing in the fossil record about 420 million years ago, long before vessel members. Most seedless vascular plants, such as ferns, and cone-bearing seed plants, such as pines, have tracheids only. Although vessel members evolved independently several times and are present in a few seedless vascular plants, vessel members are usually associated with flower-bearing seed plants.
Tracheids are less specialized than vessel members. Tracheids appear first in the fossil record; vessel members and fibers are thought to have derived from tracheids. The less-specialized tracheid provides both water-conducting and strenghtening traits in one cell. In plants with fibers and vessel members, fibers specialize in strengthening plant tissue and vessel members specialize in water conduction.
Mature tracheary elements are dead, tubelike cells. Only cell walls remain intact at the end of the differentiation process; the protoplast is completely eliminated, leaving a hollow cell. Whether tracheary elements arise from meristematic cells of the procambium or later in development from the vascular cambium, the pattern of tracheary element development and maturation is similar.
One of the first indications that a meristematic cell will become a tracheary element is cell elongation; mature tracheary elements are longer than they are wide. The elongating cells have thin, primary cell walls; but, as the cells elongate, additional cell wall compounds are deposited to the inside of the primary wall. The additional wall deposition forms the secondary cell wall. One of the secondary wall compounds is a complex polymer called lignin. A lignified cell wall is impermeable to water. To allow for water transport from cell to cell, numerous regions of the primary cell wall remain free of secondary wall deposition. The regions lacking secondary walls are called pits. Pit structure and the pattern of pitting on the walls of tracheary elements are specific for different plant species and are useful traits in plant identification.
Water Flow in Tracheary Elements.
The pits of adjacent cells are aligned with one another, allowing water to pass from tracheary element to tracheary element. Water passes through aligned pits because the two adjacent primary walls, and the middle lamella cementing the two cells together, are composed of complex carbohydrates permeable to water, such as cellulose and pectin. An aligned pair of pits is called a pit pair, and the primary walls and middle lamella of the pit pair are called a pit membrane.
Although pit membranes are permeable to water, they do offer some resistance to the flow of water between cells. In vessel members, the maturation process includes dissolution of the end walls to form perforation plates. Perforation plates are cell wall regions that are completely open, offering no resistance to water flow. Vessel members are connected end-to-end, forming tubes called vessels. Water taken into a vessel from surrounding parenchyma cells, tracheids, or other vessels must pass through pits in the lateral walls of the vessels; but, once inside a vessel, water can flow unimpeded for the length of that vessel.
Because vessel members lack end walls, moving water with less resistance, they are thought to be more efficient water-conductors than tracheids. However, there is a tradeoff. If an air bubble forms in a vessel, it can expand and fill the entire vessel. An air-filled vessel can no longer function in water transport. Because water must pass through the pit membranes of pit pairs when traveling from tracheid to tracheid, air bubbles cannot pass between adjoining tracheids. Pit membranes are effective barriers to air bubbles, trapping bubbles within a single tracheid.
Secondary Wall Reinforcement.
In primary xylem, selective secondary wall deposition creates different cell wall patterns in tracheary elements. In the first formed tracheary elements of the primary xylem, secondary wall deposition tends to occur in ringlike (called annular) or helical (spiral) bands around the cell. As the primary plant body continues to lengthen, cells with annular or helical thickenings stretch. These cells are often stretched beyond functional usefulness. Later, but still during primary growth, ladder-like (scalariform), netlike (reticulate), or pitted secondary wall patterns may develop. With increasing amounts of secondary wall deposition, tracheary elements become stronger and less resistant to stretching. Therefore, scalar-iform, reticulate, and pitted tracheary elements are generally found in organs that have ceased elongation.
The lignified secondary walls and pit pairs of tracheary elements and the perforation plates of vessel members provide for efficient water-conduction through vascular tissue. Water and dissolved minerals taken in from the external environment—usually wet soil—move upward through the xylem tissue of roots into stems and finally into leaves. Within leaves, water evaporates from cell surfaces and is lost to the environment as water vapor. Water evaporation from a plant surface is called transpiration. Water is literally pulled up tracheary elements during transpiration. The evaporation-generated pulling stretches hydrogen bonds between connected water molecules, resulting in a column of water that is under tension (negative pressure). The lignified cell walls of tracheary elements are strong enough to resist the tension, preventing inward cell collapse during water movement.
Phloem: The Food-Conducting Vascular Tissue
Phloem tissue is a complex tissue consisting of parenchyma cells, fibers, and one of two types of food-conducting cells. Sieve-tube members and sieve cells are food-conducting cells and are collectively called sieve elements. Sieve-tube members connected end-to-end form a sieve tube. Sieve cells evolved before sieve-tube members and are less specialized. Cone-bearing seed plants have sieve cells, and the more advanced flowering seed plants have sieve-tube members.
Sieve elements are long, narrow cells with primary cell walls. During differentiation, a sieve element undergoes major protoplasmic changes, including loss of its nucleus and vacuolar membrane. In addition, the cell loses ribosomes, the Golgi complex, and the cytoskeleton system, but the cell membrane remains intact. Next to the cell membrane, a network of smooth endoplasmic reticulum lines the cell, and a few plastids and mitochondria remain intact.
The defining feature of sieve elements are sieve areas. A sieve area is a cluster of pores in the wall of a sieve element. These pores allow materials to flow from cell to cell. In sieve cells, end walls overlap and, although sieve areas are found on all wall surfaces, they are concentrated on overlapping wall regions. Sieve-tube members have two types of sieve areas. Sieve plates are a specialized type of sieve area with relatively large pores. Most sieve plates occur on end walls of sieve-tube members with relatively smaller-pored sieve areas on the lateral cell walls. Sieve plate complexity is variable. Some sieve-tube members have compound sieve plates with several sieve areas on a steeply inclined end wall. Sieve-tube members with compound sieve plates are considered less specialized than sieve-tube members with simple sieve plates and horizontal end walls. The pores of simple sieve plates are relatively wider than the pores of compound plates, and wider pores increase cell-to-cell connection.
Unlike tracheary elements of the xylem, the fluid contents of sieve elements in the phloem are under positive pressure, and the movement of sugars and other substances through sieve elements is directed by pressure differences. Because cell contents are under pressure, sieve elements require a method of sealing cells to prevent the loss of cellular contents upon sieve element injury or death. One blocking substance is callose. Callose is a carbohydrate that lines the pores of sieve areas and sieve plates; it can rapidly expand, filling pores and blocking the loss of cell contents. A few flowering plant sieve-tube members contain a protein substance called P-protein. (The "P" is for phloem.) P-protein appears to line sieve plate pores in living cells and to plug pores of damaged cells. Although the function of P-protein is not known, it may serve as an additional method of blocking sieve pores and preventing the loss of cell contents upon injury.
Companion Cells and Albuminous Cells.
There are two types of specialized cells associated with sieve elements. Sieve-tube members are always associated with companion cells. Both cells arise from the same meristematic cell and are joined by numerous, well-developed plasmodesmatal connections. Companion cells probably provide a delivery and support system for the nonnucleated sieve-tube members. Associated with sieve cells are specialized parenchyma cells called albuminous cells. Albuminous cells may perform the same function for sieve cells as companion cells do for sieve-tube members.
Most organisms that we automatically classify as plants, such as roses and corn, have a vascular tissue system and are called vascular plants. However, plants such as mosses lack this highly developed transport system and are classified as nonvascular plants.
Informally, nonvascular plants are called bryophytes, and include three groups of plants: liverworts, hornworts, and mosses. Nonvascular plants specialize in absorbing moisture by efficiently moving water over their surfaces through capillary action. In many of these small plants, there is an additional internal conducting tissue that allows for efficient water and food conduction. When present, bryophyte-conducting tissue consists of two specialized cell types: water-conducting hydroids and food-conducting leptoids. Hydroids are elongated cells with thin primary cell walls and no protoplast at maturity. Leptoids are elongated cells, but their lateral cell walls are thick. The end walls of leptoids contain numerous plasmodesmata that may enlarge to form small pores. At maturity, the nuclei of leptoids degenerate. Because bryophyte conducting tissue apparently lacks lignin, it is not considered true vascular tissue.
see also Anatomy of Plants; Cells, Specialized Types; Roots; Tissues; Translocation; Water Movement.
Deborah K. Canington
Thomas L. Rost
Cutter, E. G. Plant Anatomy. Part I: Cells and Tissues, 2nd ed. Menlo Park, CA: Addison-Wesley Publishing Company, 1978.
Esau, K. Anatomy of Seed Plants, 2nd ed., New York: John Wiley & Sons, 1977.
Fahn, A. Plant Anatomy, 4th ed., Oxford: Pergamon Press, 1990.
Mauseth, James D. Plant Anatomy. Menlo Park, CA: Benjamin/Cummings Publishing Company, 1988.