Sucrose and amino acids are translocated within the living cytoplasm of the sieve tubes. 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; includes sucrose and amino acids dissolved in water Direction of transport Upwards from roots to leaves Upwards and downwards.
Type of transport. Physical process. Requires energy. Right: Cross section of the trunk of a California fan palm Washingtonia filifera showing scattered vascular bundles that appear like dark brown dots. The dot pattern also shows up in the petrified Washingtonia palm left. The pores in the petrified palm wood are the remains of vessels. The large, circular tunnel in the palm wood right is caused by the larva of the bizarre palm-boring beetle Dinapate wrightii shown at bottom of photo.
An adult beetle is shown in the next photo. Through a specialized heating process, the natural sugar in the wood is caramelized to produce the honey color.
Vascular bundles typical of a woody monocot are clearly visible on the smooth cross section. The transverse surface of numerous lignified tracheids and fibers is actually harder than maple. Much of the earth's coal reserves originated from massive deposits of carbonized plants from this era. Petrified trunks from Brazil reveal cellular details of an extinct tree fern Psaronius brasiliensis that lived about million years ago, before the age of dinosaurs.
The petrified stem of Psaronius does not have concentric growth rings typical of conifers and dicot angiosperms. Instead, it has a central stele composed of numerous arcs that represent the vascular bundles of xylem tissue. Surrounding the stem are the bases of leaves. In life, Psaronius probably resembled the present-day Cyathea tree ferns of New Zealand. A petrified trunk from the extinct tree fern Psaronius brasiliensis.
The central stele region contains arc-shaped vascular bundles of xylem tissue. The stem is surrounded by leaf bases which formed the leaf crown of this fern, similar to present-day Cyathea tree ferns of New Zealand. This petrified stem has been cut and polished to make a pair of bookends. A well-preserved stem section from the extinct tree fern Psaronius brasiliensis. Note the central stele region containing arcs of xylem tissue vascular bundles.
The structure of this stem is quite different from the concentric growth rings of conifers and dicots, and from the scattered vascular bundles of palms.
References Bailey, L. Hortus Third. Macmillan Publishing Company, Inc. Chrispeels, M. Plants, Food, and People.
Freeman and Company, San Francisco. Heiser, C. Hill, A. Economic Botany. McGraw-Hill, New York. Klein, R. Harper and Row, Publishers, New York. Langenheim, J. Plant Biology and its Relation to Human Affairs. Levetin, E. Plants and Society. Brown, Publishers, Dubuque, Iowa.
Richardson, W. The presence, quantities, and arrangements of these cell types in the tissue commonly vary and may be taxonomic informative [ 3 , 4 ]. Lists depicting these variations in all phloem cell types are of ultimate importance for complete bark descriptions [ 5 ]. What follows is a description of these three major cell types in the phloem.
General aspects of the secondary phloem. Note callose staining with resorcin blue evidencing the slightly inclined simple sieve plates. Note also the P-protein asterisk next to the sieve plate. Magnoliaceae showing sieve tube elements in clusters, with conspicuous nacreous walls, parenchyma cells p , clusters of fibers f , and rays r.
Note that no collapse is seen in the nonconducting phloem of Carya. Sieve element is a general term that encompasses all conducting cells of the phloem, both sieve cells and sieve tube elements [ 1 , 6 ]. The name sieve derives from the strainer appearance given to the cells by the presence of numerous pores crossing their bodies Figure 2c.
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. This separates plants into vascular and non-vascular plants. Most plants have xylem and phloem and are known as vascular plants but some more simple plants, such as mosses and algae, do not have xylem or phloem and are known as non-vascular plants.
Phloem and xylem are closely associated and are usually found right next to one another. Xylem tissue is used mostly for transporting water from roots to stems and leaves but also transports other dissolved compounds. Phloem is responsible for transporting food produced from photosynthesis from leaves to non-photosynthesizing parts of a plant such as roots and stems.
The phloem carries important sugars, organic compounds, and minerals around a plant. Sap within the phloem simply travels by diffusion between cells and works its way from leaves down to the roots with help from gravity.
Sieve-tube members are living cells that create chains of cells running the length of the plant. The cells of sieve-tube members are missing some important structures such as a nucleus, ribosomes and a vacuole which is where companion cells come in.
Companion cells are not lacking in any vital organelles and their nucleus and ribosomes serve both the sieve-tube member and itself. The companion cell can sometimes also deliver sugars and other substances into the sieve-tube members from neighboring cells. The xylem is responsible for keeping a plant hydrated. Two different types of cells are known to form the xylem in different plant groups: tracheids and vessel elements.
Tracheids are found in most gymnosperms , ferns , and lycophytes whereas vessel elements form the xylem of almost all angiosperms. Xylem cells are dead, elongated and hollow.
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