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. Older phloem shows more sclerification than younger phloem, and the sclerenchyma may also act as a barrier to bark attackers [ 21 ]. The sclerenchyma is typically divided in two categories: fibers and sclereids. These cell types differ mainly in form and size, but origin has also been used to distinguish them [ 26 ].
Fibers are long and slender cells, derived from meristems, the fiber primordia [ 1 , 26 , 27 ]. In the primary phloem, fiber caps are sometimes found in association with the protophloem Figure 5a and are named protophloem fibers. Since only an ontogenetic study can evidence whether these fibers indeed differentiate within the protophloem, a term coined in the nineteenth century German and American literature, pericyclic fibers, has been recommended to be used instead of primary phloem fibers or perivascular fibers [ 5 ].
In the monocotyledons, fibers are commonly an important component of the vascular bundles Figure 5b — d. Commonly these fibers are associated with the phloem Figure 5b , but they might also be associated with the xylem Figure 5c or be central in the vascular bundle Figure 5d. These fibers are not, however, understood as part of either phloem or xylem; although they are of vascular nature, they differentiate directly from procambium. Vascular fibers associated to eudicot and monocot primary structure.
Secondary phloem sp beginning to be produced. Vascular bundles in monocotyledons. Phloem in two strands around a wide metaxylem vessel. Phloem on the top side of the picture. Picture credit to Marina Blanco Cattai. Sclereids may have different forms and sizes Figure 6a — c. Within the phloem, they are more typically square or polygonal stone cells and contain numerous pits and conspicuous pit canals. Holdheid [ 26 ] defines that a sclereid is a cell derived from the belated sclerification of a parenchyma cell, and that is in fact the rule in the majority of cases Figure 6a and b.
However, there are lineages in which the sclereids differentiate very close to the cambium e. In these cases, the form is enough to define the sclereid. Sclereids in the secondary phloem. On the other hand, there are cases where long and slender cells derive from previously mature parenchyma cells and are morphologically difficult to distinguish from fibers. In these cases, these cells are called fiber sclereids and may be even in concentric layers, such as in apple trees and pears Malus domestica and Pyrus communis , respectively; [ 15 ].
Sclereids can also develop with different arrangements in the phloem, being isolated and scattered or in clusters Figures 6a — c [ 5 ]. The rays in the conducting phloem have typically the same organization in terms of width, height, and cellular composition as the secondary xylem.
In this respect the rays vary from uniseriate to multiseriate Figure 7a and may be homocellular or heterocellular Figure 7b. Homocellular rays are those composed of cells of one shape, all procumbent or all upright common in many shrubs. Heterocellular rays are those where more than one cell shape is present together Figure 7b. Ray composition is appreciated in radial sections. Rays in the secondary phloem. Fibers f in bands. Because the vascular cambium produces much more xylem to the inside than phloem to the outside, phloem rays typically greatly dilate toward the periphery of the organ Figure 7c.
It is not uncommon that a dilatation meristem longitudinal to the cambium forms in some barks Figure 7c , especially in families with very wide, wedge-like rays such as the Malvaceae. Plants with unicellular rays very rarely have dilatation by cell division [ 15 , 26 ]. Instead, they have great lateral expansion of their single cells.
Ray width can be only determined in tangential sections. Rays are typically exclusively parenchymatic; however, in many species sieve elements appear in the rays and are called ray sieve cells or radial sieve cells [ 5 , 28 , 29 ]. These cells were recorded connecting two different sieve tubes collections of sieve tube elements. Ray sieve elements seem to be present in taxa where perforated ray cells have been also recorded [ 30 ].
Similarly to the primary xylem, the primary phloem is divided in protophloem and metaphloem Figure 1d , with the protophloem differentiating first, while the plant is still elongating, and the metaphloem differentiating last.
The phloem is always exarch, independently of the organ. Protophloem sieve elements sometimes lack companion cells, such as in Arabidopsis , and in this case the sieve elements are sustained by other neighboring parenchyma cells. Commonly, the protophloem quickly becomes obliterated and loses function. In plants without secondary growth, the metaphloem will be conducted during the entire life of the plant, as in the monocotyledons Figure 5b — d [ 11 ].
Different vascular plant lineages display different arrangements of the primary xylem and phloem, depending on the stele type. Two main types of steles exist, the protostele and the siphonostele. In the protostele, the entire center of the organ is composed of vascular tissue Figure 1a , with the phloem in strands alternated with a central xylem in the protostele, haplostele, and actinostele Figure 1a , while primary phloem is interspersed in the protostele plectostele [ 6 ].
The roots of all the vascular plants are protostelic Figure 1a. The stems, however, can vary. In the lycophytes, they are always protostelic, while in the ferns monilophytes they might be protostelic, such as in Psilotum , or in all other range of siphonostelic steles [ 31 ].
The siphonostele evolved in concert with the macrophytes and resulted in the formation of a central pith derived from the ground meristem. No lineage displays as much diversity in the primary vasculature architecture as do the ferns. In the seed plants, that is, gymnosperms and angiosperms, the stem stele is always a syphonostele, either a eustele, where discrete vascular bundles form a concentric ring, or the atactostele, a type of stele exclusive of the monocotyledons where the bundles are scattered in the entire stem center.
In general, this happens between where these substances are made the sources and where they are used or stored the sinks. This means, for example, that sucrose is transported:. Applied chemicals, such as pesticides , also move through the plant by translocation. Xylem and phloem Plants have tissues to transport water, nutrients and minerals. The cells that make up the phloem are adapted to their function:. Plant transport tissues - 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. Become strengthened by a chemical called lignin. The cells are no longer alive. Lignin gives strength and support to the plant. Phloem sieve-tube elements have reduced cytoplasmic contents, and are connected by a sieve plate with pores that allow for pressure-driven bulk flow, or translocation, of phloem sap. 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.
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