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. 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 ]. We call lignified cells wood. Phloem The phloem moves food substances that the plant has produced by photosynthesis to where they are needed for processes such as: growing parts of the plant for immediate use storage organs such as bulbs and tubers developing seeds Transport in the phloem is therefore both up and down the stem. The cells that make up the phloem are adapted to their function: Sieve tubes — specialised for transport and have no nuclei.
Each sieve tube has a perforated end so its cytoplasm connects one cell to the next. Companion cells — transport of substances in the phloem requires energy. One or more companion cells attached to each sieve tube provide this energy. Enzymes 6. Cell Respiration 9. Photosynthesis 3: Genetics 1. Genes 2. Chromosomes 3. Meiosis 4. Inheritance 5. Genetic Modification 4: Ecology 1. Energy Flow 3. Carbon Cycling 4. Climate Change 5: Evolution 1. Evolution Evidence 2. Natural Selection 3.
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