Together, xylem and phloem tissues form the vascular system of plants. Xylem is the tissue responsible for supporting the plant as well as for the storage and long-distance transport of water and nutrients, including the transfer of water-soluble growth factors from the organs of synthesis to the target organs.
The tissue consists of vessel elements, conducting cells, known as tracheids, and supportive filler tissue, called parenchyma. These cells are joined end-to-end to form long tubes. Vessels and tracheids are dead at maturity. Tracheids have thick secondary cell walls and are tapered at the ends. It is the thick walls of the tracheids that provide support for the plant and allow it to achieve impressive heights. Companion cells — transport of substances in the phloem requires energy.
One or more companion cells attached to each sieve tube provide this energy. A sieve tube is completely dependent on its companion cell s. Comparison of transport in the xylem and phloem Xylem Phloem Type of transport Physical process Requires energy Substances transported Water and minerals Products of photosynthesis including sugars and amino acids dissolved in water Direction of transport Upwards Upwards and downwards. Physical process. Requires energy. Plants contain vessels which function to transport water and sugars from one part of the plant to another.
Xylem vessels transport water and dissolved mineral ions from the roots to the rest of the plant and also provide structural support. Phloem vessels transport dissolved substances, such as sucrose and amino acids , from the leaves to the rest of the plant. Xylem and phloem vessels are grouped together within the plant stem and form vascular bundles.
Sclerenchyma fibres are also found within vascular bundles and provide support to the stem. Within the plant stem , xylem vessels are found right on the inside. Phloem tissue is located in the middle of the vascular bundle and sclerenchyma fibres are found on the outside.
In the root , the xylem forms a cross-like structure in the centre which is surrounded by phloem vessels. This arrangement adds strength to the root as it pushed through the soil.
Within the leaf , the xylem vessels are found towards the top of the vascular bundle with the phloem vessels found underneath. Xylem vessels transport water and mineral ions from the roots to the rest of the plant.
They are made up of dead, hollow cells with no end cell walls. These phloem strands are initially primary, but a cambium can differentiate between the protoxylem and the phloem strands and develop secondary tissues inside of the pith. Being derived from the cambium, the secondary phloem will share a number of characteristics with the secondary xylem. For instance, it is divided in an axial and radial system. The axial system is composed of sieve elements, axial parenchyma cells, and fibers, and the radial system is formed by rays, which are typically parenchymatic Figure 2a — c.
Similar to secondary xylem, the secondary phloem can be storied Figure 7a or non-storied Figure 2b and c , depending whether the cambial mother cells are organized in tiers or not.
Some trees will have growth rings, with an early and a late phloem, both in temperate and tropical regions, but their characterization is only possible with periodical collections [ 5 ]. Sometimes, but not always, the fiber band width gives a hint on the presence of growth rings or the formation of very small sieve elements in the late phloem [ 1 , 5 ].
In conifers except Gnetales the secondary phloem is typically marked by an alternation of axial cell types Figure 3a and b , uniseriate rays, and, in many lineages, axial and radial resin canals e. In the Pinaceae , the phloem is marked by the presence of an alternation of sieve cells and bands of axial parenchyma with phenolic contents, some also with druses. In the nonconducting phloem of Pinaceae , sclereids differentiate.
In all other conifers, in addition to the alternation of parenchyma bands and sieve cells, fiber bands are present Figure 3a and b. Therefore, sieve cells, parenchyma cells with phenolic content, and bands of fibers appear in alternation in non- Pinaceae and Gnetales conifers, including Araucariaceae , Cupressaceae , Podocarpaceae , Taxaceae , and Taxodiaceae [ 8 , 21 ].
Another marked difference of these conifers compared to Pinaceae is that they contain a lot of crystals in their cell walls, including in Gnetales see New World Ephedra ; [ 36 ] , while in Pinaceae they are exclusively inside of idioblastic cells. In other gymnosperms, in particular in Gnetales and Cycads, the first remarkable difference is the presence of very wide, multiseriate rays alternating with uniseriate rays.
The wide rays in both groups have, however, evolved independently, since Cycads are a sister to all other gymnosperms, while Gnetales are within the conifers, as sister to the Pinaceae [ 31 , 37 ]. In Cyca and the extinct Cycadoidea , sieve cells and phloem parenchyma alternate with fibers, which can be in tangential bands or not [ 38 , 39 ]. In Cyca , the sieve cells appear in radial rolls [ 38 ], while in Cycadoidea there is a constant alternation of one sieve cell or phloem parenchyma to one fiber [ 39 ].
The nonconducting phloem of Cycas is marked by the collapse of sieve cells, enlargement of the axial parenchyma cells, ray dilatation, and sclerosis of some parenchymatic cells [ 38 ]. More than one ring of secondary phloem is present in some Cycads e. Within the Gnetales, in Ephedra axial parenchyma cells are interspersed with sieve cells Figure 4a , and fiber may or may not be present and are typically gelatinous [ 36 ].
In the nonconducting phloem of Ephedra , the sieve cells and Strasburger cells collapse with the enlargement of the axial and radial parenchyma cells Figure 4a with more ergastic contents [ 13 ]. In Gnetum , large areas of parenchyma sclerify, forming bands in the nonconducting phloem. The secondary phloem of Welwitschia is described as containing a large amount of fibers [ 21 ]. Within the angiosperms, the diversity of phloem cell type arrangements reaches its maximum. The structure can be storied Figure 7a or non-storied Figure 2b and c ; sclerenchyma can be present or lacking.
The rays may be uni-, bi-, or multiseriate. A large array of secretory cells may be encountered, such as resin canals, laticifers, and mucilaginous cells. Crystalliferous parenchyma is also very common, especially when associated with fibers. The variation in cell type arrangements can be of taxonomic interest.
Sieve elements can vary in morphology and arrangement. They can be solitary Figure 2f , scattered in the phloem e. The functional significance of the different arrangements is unknown to date, although this is one of the features in the phloem with the strongest phylogenetic signal.
The presence, type, and arrangements of fibers and sclereids are one of the most informative characters in the bark [ 4 ]. In Apocynaceae , the fibers are completely absent, except in Aspidosperma , the sister group of all other Apocynaceae [ 35 ]. In Aspidosperma , they can appear solitary scattered across the phloem or in clusters.
In some lineages, fibers appear in concentric alternating bands, as in Leguminosae Papilionoideae , Mimosoideae Figure 4c [ 41 ], Bignoniaceae [ 20 ], and Malvaceae , and this is a constant character among them. Phloem parenchyma more commonly constitute the background tissue in the phloem but can also be distributed in bands Figure 4b and c , radial rows, or even only around the sieve tube elements Figure 4d [ 5 ].
In a system where transport goes against the direction of transpiration, its functionality relies on the presence of a plasma membrane across the entire system to create an osmotic pressure, hence the need of a conducting system with living cells [ 44 ]. Recent studies have been refining aspects involved in the photosynthate conduction to explain long-distance transports across large trees with such a simple system [ 44 , 45 ].
A direct role of intracellular calcium has also been reported in the dissolution of nondispersive P-proteins and facilitation of transport [ 46 ]. Likely, the anatomical structure of the phloem discussed in the previous sections of this chapter will prove to play a role in the system. For instance, phloem sieve element length scale with the tree sizes and sieve plate type [ 45 ]. Across the entire pathway, sugars are removed from the system to sustain all cells in the plant body.
This mechanism is only possible with the concerted mechanism between sieve elements and their close related cells Strasburger cells and companion cells , with these accompanying cells constantly channeling substances and macromolecules toward the sieve elements [ 44 ]. The Strasburger and companion cells carry the loading and unloading of the sieve elements. Given the function of loading and unloading, the companion cell-sieve tube element size ratio is directly related to being in the source or the sink of sugars [ 44 ].
For instance, in leaves the companion cells are typically much larger, for they have the high demand of constantly loading the sieve tubes. In areas of release of the sugars unloading , the companion cells are much smaller or even absent [ 44 ]. In the economic uses, it is not always easy to distinguish the use of the phloem from that of the periderm, since both together compose the bark of a woody plant. The phloem corresponds to the inner bark, and the periderm to the outer bark.
The bark has a long history of utilization, from the production of remedies [ 49 ], aphrodisiacs yohimbe , insecticides [ 50 ], dyes, tannins [ 50 ], angostura, fibers [ 51 ], gums and resins [ 50 ], latex, and flavorings [ 52 ]. In indigenous groups from British Columbia Canada and Tanzania, barks from dozens of species of woody plants are used as carbohydrate food, medicine, fibers, and structural material [ 50 , 53 ]. The rubber tree, Hevea brasiliensis Euphorbiaceae , is known from the extraction of latex to the production of rubber.
Laticifers are present in concentric rings in the secondary phloem of the rubber tree and are an important economic asset in some tropical countries. Bark residues have also been considered for mulching [ 53 , 54 , 55 ], to build particle boards [ 56 , 57 ], as fuel, and a source of food for ruminants [ 52 ]. I would like to express gratitude to Ray F. Lima for allowing their slide collections to be photographed and Leyde N.
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