Phloem is comprised of cells called sieve-tube elements. Phloem sap travels through perforations called sieve tube plates. Neighboring companion cells carry out metabolic functions for the sieve-tube elements and provide them with energy. Lateral sieve areas connect the sieve-tube elements to the companion cells. Image credit: OpenStax Biology. This increase in water potential drives the bulk flow of phloem from source to sink.
Unloading at the sink end of the phloem tube can occur either by diffusion , if the concentration of sucrose is lower at the sink than in the phloem, or by active transport , if the concentration of sucrose is higher at the sink than in the phloem. If the sink is an area of active growth, such as a new leaf or a reproductive structure, then the sucrose concentration in the sink cells is usually lower than in the phloem sieve-tube elements because the sink sucrose is rapidly metabolized for growth.
If the sink is an area of storage where sugar is converted to starch, such as a root or bulb, then the sugar concentration in the sink is usually lower than in the phloem sieve-tube elements because the sink sucrose is rapidly converted to starch for storage.
But if the sink is an area of storage where the sugar is stored as sucrose, such as a sugar beet or sugar cane, then the sink may have a higher concentration of sugar than the phloem sieve-tube cells. In this situation, active transport by a proton-sucrose antiporter is used to transport sugar from the companion cells into storage vacuoles in the storage cells. Sucrose is actively transported from source cells into companion cells and then into the sieve-tube elements.
This reduces the water potential, which causes water to enter the phloem from the xylem. The resulting positive pressure forces the sucrose-water mixture down toward the roots, where sucrose is unloaded. Transpiration causes water to return to the leaves through the xylem vessels.
This video beginning at provides a more detailed discussion of the pressure flow hypothesis:. It should be clear that movement of sugars in phloem relies on the movement of water in phloem. But there are some important differences in the mechanisms of fluid movement in these two different vascular tissues:. It is the faith that it is the privilege of man to learn to understand, and that this is his mission.
Organismal Biology. Skip to content. In the root, the xylem forms a central column, forming a solid support. The phloem is towards the centre, outside the xylem. In the stem, the transport tissues of the xylem and phloem are grouped into vascular bundles. Plant transport tissues - xylem and phloem Plants have two transport systems - xylem and phloem. Xylem The xylem transports water and minerals from the roots up the plant stem and into the leaves. Vessels: Lose their end walls so the xylem forms a continuous, hollow tube.
This allows water to flow easily. Become strengthened by a chemical called lignin. The cells are no longer alive. Lignin gives strength and support to the vessel. Phloem Phloem moves sugar that the plant has produced by photosynthesis to where it is needed for processes such as: growing parts of the plant for immediate use storage organs such as bulbs and tubers developing seeds respiration Transport in the phloem is therefore both up and down the stem.
These pores are specialized plasmodesmata of wider diameter, and the sieve areas are basically specialized primary pit fields [ 7 ]. The sieve pores are usually lined up with callose, which were shown to be related with the formation of the sieve pores in angiosperms, although not in gymnosperms [ 8 ].
Large amounts of callose deposit in the sieve areas also when the sieve element loses conductivity, suffers injury, or becomes dormant. Callose in gymnosperms is typically wound callose [ 8 ]. Callose can be easily detected with aniline blue under fluorescence or resorcin blue [ 9 ] Figure 2b and c. Sieve elements have only primary walls, but sometimes this wall can be very thick receiving the name of nacreous walls Figure 2d [ 10 ] and can be present in all major vascular plant lineages [ 1 ].
Nacreous walls can be very thick, and some authors have proposed they would be secondary walls [ 1 , 8 ]. Nacreous walls can almost occlude the entire lumen of the sieve element Figure 2d ; hence, its presence needs to be considered in experiments of sugar translocation. Such thick walls might be related to resistance to high turgor pressures within the sieve elements.
Nacreous walls seem to have a strong phylogenetic signal and are much more common in some families, such as Annonaceae , Calycanthaceae , and Magnoliaceae [ 10 ]. There are basically two types of sieve elements: sieve cells and sieve tube elements.
The sieve tube elements are distinguished by the presence of sieve plates, that is, sieve areas with wider and more abundant sieve pores, usually in both extreme ends of the cells, while sieve cells lack sieve plates [ 1 , 6 , 8 ]. A group of connected sieve tube elements form a sieve tube [ 8 ].
According to this concept, lycophytes and ferns have sieve cells [ 1 ]. The longevity of sieve elements varies. In many species it is functional for just one growth season, while for other species they can be functional a couple of years, or in the case of plants that lack secondary growth, they will be living for the entire plant life spam. Palm trees would perhaps be the plants with the oldest conducting sieve tube elements, since some reach years [ 11 ].
In other plants, on the other hand, the sieve elements collapse a few cells away from the vascular cambium, corresponding to a fraction of the mm. In a mature tree, most of the secondary phloem will generally be composed of sieve elements no longer conducting. This region is called nonconducting phloem, in opposition to the area where sieve elements are turgid and conducting, called conducting phloem [ 5 , 8 ] Figure 2e and f.
The term collapsed and noncollapsed phloem and functional and nonfunctional phloem are not recommended, since in some plants the nonconducting phloem keeps its sieve elements intact Figure 2f , and although large parts of the phloem may not be conducting, the tissue as a whole is certainly still functioning in storage, protection, and even dividing or giving rise to new meristems, such as the phellogen and the dilatation meristem of some rays [ 5 , 8 ].
Sieve cells are typically very elongated cells with tapering ends Figure 3b , which lack sieve plates, that is, lack an area in the sieve element where the pores are of a wider diameter. Even though the sieve areas may be more abundant in the terminal parts of the sieve cells, the pores in these terminal areas are of the same diameter as those of the lateral areas of the sieve element.
Sieve cells lack P-protein in all stages of development. The sustenance of the sieve cells is carried by specialized parenchyma cells in close contact with the sieve elements, with numerous plasmodesmata, which maintain the physiological functioning of the sieve cells, including the loading and unloading of photosynthates.
These cells are known either as albuminous cells or Strasburger cells. However, because the high protein content is not always present, the name Strasburger cell, paying tribute to its discoverer Erns Strasburger, is recommended over albuminous cells [ 5 , 12 ]. Strasburger cells in the secondary phloem can be either axial parenchyma cells, as is common in Ephedra [ 13 ], or ray parenchyma cells, as is common in the conifers Figure 3c [ 14 ].
More commonly, the most conspicuous Strasburger cells in conifers are the marginal ray cells which are elongated Figure 3c and have a larger number of symplastic contact with the sieve cells [ 14 ].
Sometimes declining axial parenchyma cells also acts as Strasburger cells in Pinus [ 14 ]. The only reliable character to distinguish a Strasburger cell from an ordinary cell is the presence of conspicuous connections [ 14 ]. In the primary phloem, parenchyma cells next to the sieve cells are those which act as Strasburger cells. The secondary phloem of conifers. Longitudinal radial section LR of the secondary phloem of Sequoia sempervirens Cupressaceae showing alternating tangential bands of sieve cells, axial parenchyma, and fibers, interrupted by uniseriate rays.
Sieve pores distributed across the walls of long sieve cells. LR section of Pinus strobus Pinaceae showing the elongated marginal ray cells in close contact with the sieve cells. These are the Strasburger cells. A synapomorphy of the angiosperms is the presence of sieve tube elements and companion cells, both sister cells derived from the asymmetrical division of a single mother cell. In some instances, these mother cells can divide many times, creating assemblages of sieve tube elements and parenchyma cells ontogenetically related [ 15 ].
Sieve tube elements have specialized areas in the terminal parts of the sieve elements in which a sieve plate is present Figures 2b and c. Within the sieve plate, the pores are much wider than those of the lateral sieve areas, evidencing a specialization of these areas for conduction [ 16 ]. The protoplast of sieve tube elements contain a specific constitutive protein called P-protein P from phloem, also known as slime; Figure 2b , which in some taxa e.
Even in lineages of angiosperms where vessels were lost and tracheids re-evolved, such as Winteraceae in the Magnoliids and Trochodendraceae in the eudicots , sieve elements and companion cells are present [ 19 ], suggesting the independent evolution of these two plant vascular tissues derived from the same meristem initials.
Since the sieve tube element loses its nucleus and ribosomes, the companion cell is the cell responsible for the metabolic life of the sieve elements, including the transport of carbohydrates in and out the sieve elements [ 7 ]. Companion cells may be arranged in vertical strands, with two to more cells Figure 2b.
Other parenchyma cells around the sieve tube integrate with the companion cells and can also act in this matter [ 7 ]. Typically, the cells closely related with the sieve tube elements die at the same time as the sieve element loses conductivity.
Sieve tube elements vary morphologically. The sieve plates can be transverse to slightly inclined Figure 2b or very inclined Figure 2c and contain a single sieve area Figure 2b or many Figure 2c. When one sieve area is present, the sieve plate is named simple sieve plate, while when two to many are present, the sieve plates are called compound sieve plates. Compound sieve plates typically occur in sieve tube elements with inclined to very inclined sieve plates Figure 2c.
In addition, sieve elements with compound sieve plates are typically longer than those with simple sieve plates. Evolution to sieve elements of both sieve area types has been recorded in certain lineages, such as in Arecaceae , Bignoniaceae , and Leguminosae [ 5 , 20 ], and to the present it is not still clear why the evolution of distinct morphologies would be or not beneficial.
The only clear pattern is that compound sieve plates appear in long sieve elements [ 1 ], and phloem with a lot of fibers generally has compound sieve plates [ 20 ]. In the primary phloem, just one type of parenchyma is present and typically intermingles with the sieve elements Figure 1d.
In the secondary structure, there are two types of parenchyma: axial parenchyma and ray parenchyma Figures 2b , c , 3b , c , derived, respectively, from the fusiform and ray initials of the cambium. The axial parenchyma in conifers commonly is arranged in concentric, alternating layers Figure 3a and b. These parenchyma cells contain a lot of phenolic substances, which were viewed as a defense mechanism against bark attackers [ 21 ]. In Gnetales, the phloem axial parenchyma appears to be intermingling with the sieve cells Figure 4a [ 22 ].
Some of these axial parenchyma cells act as Strasburger cells [ 13 ]. Phloem axial parenchyma distribution in secondary phloem. Six to five cells away from the cambium, the sieve cells already lose conductivity and collapse with axial parenchyma cells enlarging top arrow. There are also other parenchyma cells with less content dispersed in the phloem. Note also the fibers in concentric bands. The tissue background corresponds to the fibers. In angiosperms, the distribution of the axial phloem parenchyma is more varied, and it may appear as a background tissue where other cells are dispersed or may be in bands Figure 4b and c and radial rows or sieve-tube-centric Figure 4d [ 5 , 20 ].
The distribution of axial phloem parenchyma is commonly related to the abundance of fibers or sclereids. In species with more fibers, it is common to have a more organized arrangement of the parenchyma. For example, in Robinia pseudoacacia Leguminosae there are parenchyma bands in either side of the concentric fiber bands Figure 4c.
When very large quantities of sclerenchyma are present, such as in the secondary phloem of Carya Juglandaceae or in Fridericia , Tanaecium , Tynanthus , and Xylophragma Bignoniaceae , the sieve-tube-centric parenchyma appears Figure 4c and, as the name suggests, is surrounding the sieve tubes [ 8 , 20 , 23 ]. Although collectively described and referred to as axial phloem parenchyma, it is important to note that in many plants there will be distinct groups of phloem parenchyma within the phloem with quite different ergastic contents and therefore presumed different functions.
Some of these specialized parenchyma cells may be considered secretory structures. Within a single plant, it is not uncommon that while some cells have crystals especially when in contact with sclerenchyma , others have tannins, starch, and other substances.
In apple trees Malus domestica , Rosaceae three types of axial parenchyma have been recorded: 1 crystal-bearing cells, 2 tannin- and starch-containing cells, and 3 those with no tannin or starch, which integrate with the companion cells [ 15 ]. Within bands of axial parenchyma, canals with a clear epithelium may be formed in many plant groups such as Pinaceae , Anacardiaceae , Apiales , a feature with strong phylogenetic signal.
Some phloem parenchyma cells also act in the sustenance and support of the sieve elements, even when not derived from the same mother cell [ 7 ]. In longitudinal section, the axial phloem parenchyma may appear fusiform not segmented or in two up to several cells per strand [ 5 ].
While the phloem ages and moves away from the cambium, its structure dramatically change, and typically axial parenchyma cells enlarge Figures 4a and b , 6c , divide, and store more ergastic contents toward the nonconducting phloem. In plants with low fiber content, the dilatation undergone by the parenchyma cells typically provokes the collapse of the sieve elements. The axial parenchyma in the nonconducting phloem can dedifferentiate and give rise to new lateral meristems. In plants with multiple periderms, typically new phellogens are formed within the secondary phloem, compacting within the multiple periderms large quantities of dead, suberized phloem.
In plants with variant secondary growth, especially lianas, new cambia might differentiate from axial phloem parenchyma cells [ 24 ]. In the Asian Tetrastigma Vitaceae , new cambia were recorded differentiating from primary phloem parenchyma cells [ 25 ].
Sclerenchymatic cells are those with thick secondary walls, commonly lignified. Sclerenchyma can be present or not in the phloem, and when present it typically gives structure to the tissue. For instance, a phloem with concentric layers of sclerenchyma cells is called stratified Figures 2e , 3a , and 4c [ 5 ]—not to be confused with storied, regarding the organization of the elements in tangential section. In Leguminosae, bands of phloem are associated to the concentric fiber bands Figure 4c.
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