Plant Cells, Tissues, and Tissue Systems
Plants, like animals, have a division of labor between their different cells, tissues, and tissue systems. In this section we will examine the three different tissue systems (dermal, ground, and vascular) and see how they function in the physiology of a plant.
One of the best web pages I have found was done by Dr. David T. Webb,Assistant Professor of Botany, University of Hawaii at Manoa, Botany Department. He has some magnificent pictures and descriptions of specific plant cells. If you truly wish to see the inner beauty of plants, please visit Dr. Webb's page. You will notice that I have used some of Dr. Webb's pictures on this page for convenience, but I strongly recommend that you visit Dr. Webb's page for the full story on plant cells.
This section starts with a table showing all of the cells, tissues, and tissue systems. Following the table are detailed descriptions and illustrations.
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|Tissue||Cell Types and Their Functions|
|Periderm||Cork cells (phellem) and Parenchyma cells (pheloderm): Protection|
|Ground||Parenchyma||Parenchyma cells: photosynthesis, storage, support|
Laticifers: Secretion, storage of secondary metabolites
Oil Ducts: secretion
Resin & Gum Duct epithelial cells: secretion
|Collenchyma||Collenchyma cells:photosynthesis, support, gravity perception in grasses|
|Sclerenchyma||Fibers: support, protection|
Sclereids: support, protection
|Vascular||Xylem||Vessel Members: Water & mineral transport, support. Mostly in advanced angiosperms|
Tracheids: Water & mineral transport, support. Mostly in gymnosperms and lower angiosperms
Sclerenchyma Cells: Fibers & Sclereids: support, protection
Xylem Parenchyma Cells: storage, short distance transport
Laticifers: secretion, storage of secondary metabolites
Resin & Gum Duct Epithelial Cells: secretion
|Phloem||Sieve Tube Members & Companion Cells: transport of sugars, organic nitrogen compounds, and growth regulators in angiosperms|
Sieve cells, Albuminous Cells: transport of sugars, organic nitrogen compounds, and growth
Sclerenchyma Cell: Fibers, Sclereids: support, protection
Phloem Parenchyma Cells: storage and short-distance transport
Laticifers: secretion and storage of secondary metabolites
Resin & Gum Duct Epithelial Cells: secretion
Dermal Tissue System:
The dermal tissue system makes up the outside covering of the plant. This system consists of the epidermis and the periderm.
The epidermis consists of a single layer of cells that covers the majority of young plants. The epidermis is present throughout the life of plants that exhibit only primary growth. Primary growth refers to cells and tissues that originate from the apical meristems. Apical meristems are regions of dividing cells located at the tips of stems and roots.
Parenchyma Cells: Living, thin walled cells. Variable in shape. Lack chloroplasts. These cells cover the outside surface of herbaceous plants. Little gas exchange occurs through these cells, due to a thick covering of a lipid compound call cutin.
Sclerechyma Cells: Parenchyma cells that have developed secondary cell walls. There are two main types: fibers and sclereids. Fibers are long and narrow. They protect and support other tissues due to their thick lignified cell walls.
Stomata (pl.), Stoma (sing.): Small openings scattered throughout the epidermis. Stomata are important for gas exchange and transpiration. Each stoma is surrounded by two guard cells which contain chloroplasts. The guard cells control the size of the stomatal opening, and thus control the amount of gas exchange and transpiration.
Trichomes: These are small hairs on the plant surface. They are epidermal extensions that can alter the boundary layer over a leaf surface.. Trichomes function in light piping (concentrating light on the underlying tissues). They aid in reducing water loss through transpiration. They can also alter heat loss from a plant, and act in storage and secretion of secondary metabolites. Root hairs are also trichomes that aid in water and mineral absorption.
When plants increase in girth due to secondary growth, they slough off their epidermal tissues and replace them with periderm. The periderm is composed of cork cells (phellem) that have thick walls impregnated with suberin (a waxy substance which protects and waterproofs the surface of the cells). Cork cells are not very strong, and therefor are continually added to the plant as it grows. Periderm may also contain unsuberized,thin-walled parenchyma cells call phelloderm. Water and gas exchange occurs through openings called lenticles. Although the function of lenticles is the same as the stomata, lenticles cannot control the size of their openings. The corks found in wine bottles are cut from the bark of Quercus suber. In order to prevent wine bottle corks from leaking, they are cut at right angles to the lenticles
Ground Tissue System:
Ground tissue consists of all tissues not included in the Dermal and Vascular Tissue Systems. Ground tissue has a wide variety of functions, even though it is composed of fairly simple tissue types.
The most abundant, diverse, and versatile cells in a plant are found in the parenchyma tissue. Parenchyma cells have thin cell walls, and their structure is somewhat non-descript, but tend to be more or less isodiametric (equal diameters in all directions). What distinguishes these cells are their many and varied functions. Some examples are:
|Starch storage tissues of tubers: contain a large amount of amyloplasts (organelles where starch is stored).|
Transfer Cells: rapid transport of food metabolites associated with veins of leaves and nectaries of flowers.
Stellate Parenchyma Cells: found in ground tissue in aquatic plants that are composed of star-shaped cells with large intercellular spaces between the arms used as air canals.
Water storage cells: the stems of cacti have cells within the cortex that store large amounts of water.
Chlorenchma Cells: found in the mesophyll of leaves contain large amounts of chloroplasts.
Collenchyma cells differentiate from parenchyma cells and are alive at maturity. Collenchyma cells have uneven thickenings in their primary cell walls. Collenchyma cells are important for support of the growing regions of shoots, roots, and leaves. They are found in expanded leaves, petioles, and near the apex of stems. Adaptations of collenchyma cells that aid in their support function are: (1) ability to stretch due to their nonlignified cell walls, (2) elongated or cylindrical structure which maximizes support.
Sclerenchyma have thick, nonelastic secondary cell walls and are dead at maturity. Sclerenchyma cells support and strengthen nonexpanding tissues of the plant such as mature roots, stems, and leaves. There are two types of sclerenchyma cells, sclereids and fibers, which are distinguished by their shape and grouping. Sclereids are variable in shape, are short, and exist singularly or in small groups. Fibers are elongated and slender and exist either singularly or in bundles.
|Sclereids: occur throughout a plant. They are responsible for hard seed coats, and hulls of pea pods. Sclereids are found in the flesh of pears where they give the gritty texture.|
Fibers: originally differentiate from parenchyma cells after their extension. Fibers are classified in several ways. Commonly, fibers are classified according to their location within the plant. For example, xylem fibers or phloem fibers. Commercially, fibers are classified according to their strength. For example, hard fibers (ones that contain large amounts of lignin - usually from associated xylem cells), and soft fibers (ones that do not contain lignin). Hard fibers such as jute (from Corchorus capsularis), hemp (from Musa textilis, Furcraea gigantea, Cannabis sative), and sisal (from Agave sisalana) are used for making ropes, cords, and twines. Soft fibers such as flax (form Linumusitatissimum) are used for making linen, and also ramie (form Boehmeria nivea) which is also used for making textiles. Cotton, however, is not a sclerenchyma fiber. Cotton is formed from elongated epidermal cells that form from trichomes on the surfaces of seed coats.
Vascular Tissue System:
The vascular tissue system is important in transport. The vascular tissue system is composed of the xylem (transport of water and dissolved minerals) and phloem (transport of food - usually sucrose and other sugars-, nitrogen containing compounds, and hormones). The xylem and phloem in the primary plant body are usually closely associated in the form of vascular bundles. In woody plants the xylem forms the wood of trunks and branches as well as the central core of the roots. The bark of a tree is a mixture of old, nonfunctional phloem and the young functional phloem (periderm).
Xylem: There are two types of conducting cells in xylem, tracheids and vessel elements. Both have thick lignified secondary walls and are dead at maturity. These cells create hollow cylinders that have high tensile strength. Materials moving within the xylem are under tension. Therefor the high tensile strength of the xylem cells keeps them from collapsing. Transport in the xylem occurs in one direction = roots->stems->leaves.
|Tracheids: long, slender cells with overlapping, tapered ends. Water moves between tracheid cells via the bordered pits. Bordered pits are thin areas in the cell walls where only primary cell wall material has been deposited. Tracheids are the more primitive (less specialized) of the two xylem cells. They are found in most woody, nonflowering plants.|
Vessel Elements: short, wide cells arranged end to end. Their end walls are partially or wholly dissolved allowing them to form long, hollow tubes up to 3 meters long. The larger diameter and lack of end walls allows vessel elements to transport water more rapidly. Vessel element are evolutionarily more advanced than tracheids. They are found in angiosperms and are one of the major reasons why angiosperms the dominant land plant.
Xylem Fibers and Xylem Parenchyma: Fibers lend support to the woody tissues (especially in plants with tracheids) while the parenchyma cells function to store metabolites, or function in secretion (resin ducts and laticifers).
Phloem: Phloem transports dissolved organic material throughout the plant. Transport within the phloem is from source to sink. This means that the direction of movement of materials within the phloem may change over time. This movement depends on the time of year and age of the plant. Phloem cells are alive at maturity, mainly because movement of materials within the phloem requires energy. Also, the materials moving within the phloem are under pressure, which means that the cell walls of the phloem cells do not have to have as great a tensile strength.
|Sieve Cells: more primitive phloem conducting cells of ferns and conifers. Sieve cells are long and tapered with overlapping ends. They have sieve areas, fields of pores scattered over their cell wall surface. These areas allow direct contact between the protoplasts of adjacent cells. The pores are surrounded by callose, a complex carbohydrate that can block the pore opening after injury. Associated with the sieve cells are Albuminous Cells that play a role in aiding the movement of materials within the phloem.|
Sieve Tube Members: more advance phloem conducting cells of angiosperms. Sieve tube members are short and wide, and arranged end to end into sieve tubes. The sieve pores are large and are concentrated along the end walls of adjacent sieve tube members. These specializations allow solutes to move more rapidly in sieve tube members and sieve cells. At maturity the nuclei in the sieve tube members disintegrates, the ribosomes disappear, and the tonoplast (vacuole membrane) breaks down. Mitochondria and plastids are still present. Sieve Tube Members are always associated with Companion Cells which control the metabolism of the cells. These two cells are connected by numerous plasmodesmata. The companion cells aid in the movement of materials into and out of the sieve tube members. Sieve tube members also contain P-protein, which stands for Phloem-protein. This protein is located along the longitudinal walls of the cells. Some sieve tube members also contain a glucose polymer called callose. Both P-protein and callose are responsible for sealing wounds in the sieve tubes.
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The ground tissue of plants includes all tissues that are neither dermal nor vascular. It can be divided into three types based on the nature of the cell walls. 1) Parenchyma cells have thin primary walls and usually remain alive after they become mature. Parenchyma forms the "filler" tissue in the soft parts of plants.usually present in cortex, pericycle, pith, and medullary rays in primary stem and root. 2) Collenchyma cells have thin primary walls with some areas of secondary thickening. Collenchyma provides extra mechanical and structural support, particularly in regions of new growth. 3) Sclerenchyma cells have thick lignified secondary walls and often die when mature. Sclerenchyma provides the main structural support to a plant.
Parenchyma (; from Greek παρέγχυμα parenkhyma, "visceral flesh" from παρεγχεῖν parenkhein, "to pour in" from παρα- para-, "beside", ἐν en-, "in" and χεῖν khein, "to pour") is a versatile ground tissue that generally constitutes the "filler" tissue in soft parts of plants. It forms, among other things, the cortex and pith of stems, the cortex of roots, the mesophyll of leaves, the pulp of fruits, and the endosperm of seeds. Parenchyma cells are living cells and may remain meristematic at maturity—meaning that they are capable of cell division if stimulated. They have thin but flexible cellulosecell walls, and are generally polyhedral when close-packed, but can be roughly spherical when isolated from their neighbours. They have large central vacuoles, which allow the cells to store and regulate ions, waste products, and water. Tissue specialised for food storage is commonly formed of parenchyma cells.
Parenchyma cells have a variety of functions:
- Their main function is to repair.
- In leaves, they form the mesophyll and are responsible for photosynthesis and the exchange of gases, parenchyma cells in the mesophyll of leaves are specialised parenchyma cells called chlorenchyma cells (parenchyma cells with chloroplasts).
- Storage of starch, protein, fats, oils and water in roots, tubers (e.g. potatoes), seed endosperm (e.g. cereals) and cotyledons (e.g. pulses and peanuts)
- Secretion (e.g. the parenchyma cells lining the inside of resin ducts)
- Wound repair and the potential for renewed meristematic activity
- Other specialised functions such as aeration (aerenchyma) provides buoyancy and helps aquatic plants in floating.
- Chlorenchyma cells carry out photosynthesis and manufacture food.
The shape of parenchyma cells varies with their function. In the spongy mesophyll of a leaf, parenchyma cells range from near-spherical and loosely arranged with large intercellular spaces, to branched or stellate, mutually interconnected with their neighbours at the ends of their arms to form a three-dimensional network, like in the red kidney bean Phaseolus vulgaris and other mesophytes. These cells, along with the epidermalguard cells of the stoma, form a system of air spaces and chambers that regulate the exchange of gases. In some works the cells of the leaf epidermis are regarded as specialised parenchymal cells, but the modern preference has long been to classify the epidermis as plant dermal tissue, and parenchyma as ground tissue.
Shapes of parenchyma:
- Stellate (found in stem of plants and have well developed air spaces between them)
- Elongated (found in pallisade tissue of leaf)
- Lobed (found in spongy and pallisade mesophyyll tissue of some plants)
The first use of "collenchyma" () was by Link (1837) who used it to describe the sticky substance on Bletia (Orchidaceae) pollen. Complaining about Link's excessive nomenclature, Schleiden (1839) stated mockingly that the term "collenchyma" could have more easily been used to describe elongated sub-epidermal cells with unevenly thickened cell walls.
Collenchyma tissue is composed of elongated cells with irregularly thickened walls. They provide structural support, particularly in growing shoots and leaves. Collenchyma tissue makes up things such as the resilient strands in stalks of celery. Collenchyma cells are usually living, and have only a thick primary cell wall made up of cellulose and pectin. Cell wall thickness is strongly affected by mechanical stress upon the plant. The walls of collenchyma in shaken plants (to mimic the effects of wind etc.), may be 40–100% thicker than those not shaken.
There are four main types of collenchyma:
- Angular collenchyma (thickened at intercellular contact points)
- Tangential collenchyma (cells arranged into ordered rows and thickened at the tangential face of the cell wall)
- Annular collenchyma (uniformly thickened cell walls)
- Lacunar collenchyma (collenchyma with intercellular spaces)
Collenchyma cells are most often found adjacent to outer growing tissues such as the vascular cambium and are known for increasing structural support and integrity.
The term "sclerenchyma" (originally Sclerenchyma) was introduced by Mettenius in 1865.
Sclerenchyma is the tissue which makes the plant hard and stiff. Sclerenchyma is the supporting tissue in plants. Two types of sclerenchyma cells exist: fibers and sclereids. Their cell walls consist of cellulose, hemicellulose and lignin. Sclerenchyma cells are the principal supporting cells in plant tissues that have ceased elongation. Sclerenchyma fibers are of great economic importance, since they constitute the source material for many fabrics (e.g. [flax]) hemp, jute, and ramie).
Unlike the collenchyma, mature sclerenchyma is composed of dead cells with extremely thick cell walls (secondary walls) that make up to 90% of the whole cell volume. The term sclerenchyma is derived from the Greek σκληρός (sklērós), meaning "hard." It is the hard, thick walls that make sclerenchyma cells important strengthening and supporting elements in plant parts that have ceased elongation. The difference between fibers and sclereids is not always clear: transitions do exist, sometimes even within the same plant.
Fibers or bast are generally long, slender, so-called prosenchymatous cells, usually occurring in strands or bundles. Such bundles or the totality of a stem's bundles are colloquially called fibers. Their high load-bearing capacity and the ease with which they can be processed has since antiquity made them the source material for a number of things, like ropes, fabrics and mattresses. The fibers of flax (Linum usitatissimum) have been known in Europe and Egypt for more than 3,000 years, those of hemp (Cannabis sativa) in China for just as long. These fibers, and those of jute (Corchorus capsularis) and ramie (Boehmeria nivea, a nettle), are extremely soft and elastic and are especially well suited for the processing to textiles. Their principal cell wall material is cellulose.
Contrasting are hard fibers that are mostly found in monocots. Typical examples are the fiber of many grasses, agaves (sisal: Agave sisalana), lilies (Yucca or Phormium tenax), Musa textilis and others. Their cell walls contain, besides cellulose, a high proportion of lignin. The load-bearing capacity of Phormium tenax is as high as 20–25 kg/mm², the same as that of good steel wire (25 kg/ mm²), but the fibre tears as soon as too great a strain is placed upon it, while the wire distorts and does not tear before a strain of 80 kg/mm². The thickening of a cell wall has been studied in Linum. Starting at the centre of the fiber, the thickening layers of the secondary wall are deposited one after the other. Growth at both tips of the cell leads to simultaneous elongation. During development the layers of secondary material seem like tubes, of which the outer one is always longer and older than the next. After completion of growth, the missing parts are supplemented, so that the wall is evenly thickened up to the tips of the fibers.
Fibers usually originate from meristematic tissues. Cambium and procambium are their main centers of production. They are usually associated with the xylem and phloem of the vascular bundles. The fibers of the xylem are always lignified, while those of the phloem are cellulosic. Reliable evidence for the fibre cells' evolutionary origin from tracheids exists. During evolution the strength of the tracheid cell walls was enhanced, the ability to conduct water was lost and the size of the pits was reduced. Fibers that do not belong to the xylem are bast (outside the ring of cambium) and such fibers that are arranged in characteristic patterns at different sites of the shoot.
Main article: Sclereid
Sclereids are a reduced form of sclerenchyma cells with highly thickened, lignified walls. These have a shape of a star.
They are small bundles of sclerenchyma tissue in plants that form durable layers, such as the cores of apples and the gritty texture of pears (Pyrus communis). Sclereids are variable in shape. The cells can be isodiametric, prosenchymatic, forked or elaborately branched. They can be grouped into bundles, can form complete tubes located at the periphery or can occur as single cells or small groups of cells within parenchyma tissues. But compared with most fibres, sclereids are relatively short. Characteristic examples are brachysclereids or the stone cells (called stone cells because of their hardness) of pears and quinces (Cydonia oblonga) and those of the shoot of the wax plant (Hoya carnosa). The cell walls fill nearly all the cell's volume. A layering of the walls and the existence of branched pits is clearly visible. Branched pits such as these are called ramiform pits. The shell of many seeds like those of nuts as well as the stones of drupes like cherries and plums are made up from sclereids.
These structures are used to protect other cells.
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Cross section of a leaf showing various ground tissue types
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