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.
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 ]. In the root, the epidermis aids in absorption of water and minerals.
Root hairs , which are extensions of root epidermal cells, increase the surface area of the root, greatly contributing to the absorption of water and minerals. A waxy substance is present on the walls of the endodermal cells. This waxy region, known as the Casparian strip , forces water and solutes to cross the plasma membranes of endodermal cells instead of slipping between the cells. In fact, endodermis is a specialized type of ground tissue.
This error is corrected below in the section about ground tissue. In the stem and leaves, epidermal cells are coated in a waxy substance called a cuticle which prevents water loss through evaporation. The cuticle is NOT present on root epidermis and is the same as the Casparian strip, which is present in the roots. To permit gas exchange for photosynthesis and respiration, the epidermis of the leaf and stem also contains openings known as stomata singular: stoma. Two cells, known as guard cells , surround each leaf stoma, controlling its opening and closing and thus regulating the uptake of carbon dioxide and the release of oxygen and water vapor.
Stems and leaves may also have trichomes , hair-like structures on the epidermal surface, that help to reduce transpiration the loss of water by aboveground plant parts , increase solar reflectance, and store compounds that defend the leaves against predation by herbivores.
Visualized at x with a scanning electron microscope, several stomata are clearly visible on a the surface of this sumac Rhus glabra leaf. At 5,x magnification, the guard cells of b a single stoma from lyre-leaved sand cress Arabidopsis lyrata have the appearance of lips that surround the opening. In this c light micrograph cross-section of an A. Wise; part c scale-bar data from Matt Russell. Trichomes give leaves a fuzzy appearance as in this a sundew Drosera sp. Leaf trichomes include b branched trichomes on the leaf of Arabidopsis lyrata and c multibranched trichomes on a mature Quercus marilandica leaf.
Wise; scale-bar data from Matt Russell. Just like in animals, vascular tissue transports substances throughout the plant body.
But instead of a circulatory system which circulates by a pump the heart , vascular tissue in plants does not circulate substances in a loop, but instead transports from one extreme end of the plant to the other eg, water from roots to shoots. Vascular tissue in plants is made of two specialized conducting tissues: xylem , which conducts water, and phloem , which conducts sugars and other organic compounds.
A single vascular bundle always contains both xylem and phloem tissues. Unlike the animal circulatory system, where the vascular system is composed of tubes that are lined by a layer of cells, the vascular system in plants is made of cells — the substance water or sugars actually moves through individual cells to get from one end of the plant to the other.
Xylem tissue transports water and nutrients from the roots to different parts of the plant, and includes vessel elements and tracheids , both of which are tubular, elongated cells that conduct water. Tracheids are found in all types of vascular plants, but only angiosperms and a few other specific plants have vessel elements. Tracheids and vessel elements are arranged end-to-end, with perforations called pits between adjacent cells to allow free flow of water from one cell to the next.
They have secondary cell walls hardened with lignin , and provide structural support to the plant. Tracheids and vessel elements are both dead at functional maturity, meaning that they are actually dead when they carry out their job of transporting water throughout the plant body. Phloem tissue, which transports organic compounds from the site of photosynthesis to other parts of the plant, consists of sieve cells and companion cells.
Sieve cells conduct sugars and other organic compounds, and are arranged end-to-end with pores called sieve plates between them to allow movement between cells. They are alive at functional maturity, but lack a nucleus, ribosomes, or other cellular structures. Sieve cells are thus supported by companion cells, which lie adjacent to the sieve cells and provide metabolic support and regulation. The xylem and phloem are always next to each other. In stems, the xylem and the phloem form a structure called a vascular bundle ; in roots, this is termed the vascular stele or vascular cylinder.
This light micrograph shows a cross section of a squash Curcurbita maxima stem. Each teardrop-shaped vascular bundle consists of large xylem vessels toward the inside and smaller phloem cells toward the outside. Xylem cells, which transport water and nutrients from the roots to the rest of the plant, are dead at functional maturity.
Phloem cells, which transport sugars and other organic compounds from photosynthetic tissue to the rest of the plant, are living. The vascular bundles are encased in ground tissue and surrounded by dermal tissue. Ground tissue cells include parenchyma, photosynthesis in the leaves, and storage in the roots , collenchyma shoot support in areas of active growth , and schlerenchyma shoot support in areas where growth has ceased. Parenchyma are the most abundant and versatile cell type in plants.
They have primary cell walls which are thin and flexible, and most lack a secondary cell wall. Parenchyma cells are totipotent, meaning they can divide and differentiate into all cell types of the plant, and are the cells responsible for rooting a cut stem. Most of the tissue in leaves is comprised of parenchyma cells, which are the sites of photosynthesis, and parenchyma cells in the leaves contain large quantities of chloroplasts for phytosynthesis.
In roots, parenchyma are sites of sugar or starch storage, and are called pith in the root center or cortex in the root periphery. Parenchyma can also be associated with phloem cells in vascular tissue as parenchyma rays. Collenchyma , like parenchyma, lack secondary cell walls but have thicker primary cells walls than parenchyma.
They are long and thin cells that retain the ability to stretch and elongate; this feature helps them provide structural support in growing regions of the shoot system.
They are highly abundant in elongating stems. Schlerenchyma cells have secondary cell walls composed of lignin , a tough substance that is the primary component of wood. Schelrenchyma cells therefore cannot stretch, and they provide important structural support in mature stems after growth has ceased.
Interestingly, schlerenchyma cells are dead at functional maturity. Schlerenchyma give pears their gritty texture, and are also part of apple cores. We use sclerenchyma fibers to make linen and rope. This waxy region, known as the Casparian strip, forces water and solutes to cross the plasma membranes of endodermal cells instead of slipping between the cells. A cross section of a leaf showing the phloem, xylem, sclerenchyma and collenchyma, and mesophyll.
Each plant organ contains all three tissue types, with different arrangements in each organ. There are also some differences in how these tissues are arranged between monocots and dicots, as illustrated below:. In dicot roots, the xylem and phloem of the stele are arranged alternately in an X shape, whereas in monocot roots, the vascular tissue is arranged in a ring around the pith. This difference in cell size and density produces the conspicuous, concentric annual rings in these woods.
Because of the density of the wood, angiosperms are considered hardwoods, while gymnosperms, such as pine and fir, are considered softwoods. See Article About Hardwoods See Specific Gravity Of Wood T he following illustrations and photos show American basswood Tilia americana , a typical ring-porous hardwood of the eastern United States: A cross section of the stem of basswood Tilia americana showing large pith, numerous rays, and three distinct annual rings.
The large spring xylem cells are vessels. In the tropical rain forest, relatively few species of trees, such as teak, have visible annual rings. The difference between wet and dry seasons for most trees is too subtle to make noticeable differences in the cell size and density between wet and dry seasonal growth.
According to Pascale Poussart, geochemist at Princeton University, tropical hardwoods have "invisible rings. Their team used X-ray beams at the Brookhaven National Synchrotron Light Source to look at calcium taken up by cells during the growing season. There is clearly a difference between the calcium content of wood during the wet and dry seasons that compares favorably with carbon isotope measurements.
The calcium record can be determined in one afternoon at the synchrotron lab compared with four months in an isotope lab. Poussart, P. Geophysical Research Letters 3: L Anatomy Of Monocot Stems M onocot stems, such as corn, palms and bamboos, do not have a vascular cambium and do not exhibit secondary growth by the production of concentric annual rings. They cannot increase in girth by adding lateral layers of cells as in conifers and woody dicots. Instead, they have scattered vascular bundles composed of xylem and phloem tissue.
Each bundle is surrounded by a ring of cells called a bundle sheath. The structural strength and hardness of woody monocots is due to clusters of heavily lignified tracheids and fibers associated with the vascular bundles. The following illustrations and photos show scattered vascular bundles in the stem cross sections of corn Zea mays : A cross section of the stem of corn Zea mays showing parenchyma tissue and scattered vascular bundles.
The large cells in the vascular bundles are vessels. This primary growth is due to a region of actively dividing meristematic cells called the "primary thickening meristem" that surrounds the apical meristem at the tip of a stem. In woody monocots this meristematic region extends down the periphery of the stem where it is called the "secondary thickening meristem.
The massive trunk of this Chilean wine palm Jubaea chilensis has grown in girth due to the production of new vascular bundles from the primary and secondary thickening meristems. Palm Wood T he scattered vascular bundles containing large porous vessels are very conspicuous in palm wood. In fact, the vascular bundles are also preserved in petrified palm.
Cross section of the trunk of the native California fan palm Washingtonia filifera showing scattered vascular bundles. The large cells pores in the vascular bundles are vessels. The palm was washed down the steep canyon during the flash flood of September The fibrous strands are vascular bundles composed of lignified cells. 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.
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