Typical leaves are attached to the plant stem by a petiole , though there are also leaves that attach directly to the plant stem. The edge of the leaf is called the margin. Illustration shows the parts of a leaf. The petiole is the stem of the leaf. The midrib is a vessel that extends from the petiole to the leaf tip. Veins branch from the midrib. The lamina is the wide, flat part of the leaf. The margin is the edge of the leaf. Leaves may be simple or compound. In simple leaves, the lamina is continuous. The a banana plant Musa sp.
In compound leaves, the lamina is separated into leaflets. Compound leaves may be palmate or pinnate. In b palmately compound leaves, such as those of the horse chestnut Aesculus hippocastanum , the leaflets branch from the petiole.
In c pinnately compound leaves, the leaflets branch from the midrib, as on a scrub hickory Carya floridana. The d honey locust has double compound leaves, in which leaflets branch from the veins. The thickness, shape, and size of leaves are adapted to specific environments. Each variation helps a plant species maximize its chances of survival in a particular habitat. Coniferous plant species that thrive in cold environments, like spruce, fir, and pine, have leaves that are reduced in size and needle-like in appearance.
In hot climates, plants such as cacti have leaves that are reduced to spines, which in combination with their succulent stems, help to conserve water. Content below adapted from OpenStax Biology Plant tissue systems fall into one of two general types: meristematic tissue , and permanent or non-meristematic tissue. Cells of the meristematic tissue are found in meristems , which are plant regions of continuous cell division and growth analogous to stem cells in animals. Meristematic tissue cells are either undifferentiated or incompletely differentiated, and they continue to divide and contribute to the growth of the plant.
In contrast, permanent tissue consists of plant cells that are no longer actively dividing. Meristems produce cells that quickly differentiate, or specialize, and become permanent tissue. Such cells take on specific roles and lose their ability to divide further. They differentiate into three main tissue types: dermal, vascular, and ground tissue.
Each plant organ roots, stems, leaves contains all three tissue types:. Each plant organ contains all three tissue types. Koning, Ross E. Plant Basics. Plant Physiology Information Website. Reprinted with permission. Before we get into the details of plant tissues, this video provides an overview of plant organ structure and tissue function:. Secondary cell walls are inflexible and play an important role in plant structural support.
Each plant tissue type is comprised of specialize cell types which carry out vastly different functions:. 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 called suberin 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. 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. 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 both dead at functional maturity, meaning that they are actually dead when they carry out their job of transporting water throughout the plant body. 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.
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The xylem and phloem always lie adjacent 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. 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. Leaves typically contains two types of parenchyma cells: the palisade parenchyma and spongy parenchyma.
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The palisade parenchyma also called the palisade mesophyll has column-shaped, tightly packed cells. Below the palisade parenchyma are the cells of the spongy parenchyma or spongy mesophyll , which are loosely arranged with air spaces that all gaseous exchange between the leaf and the outside atmosphere. Both of these types of parenchyma cells contain large quantities of chloroplasts for phytosynthesis. Parenchyma can also be associated with phloem cells in vascular tissue as parenchyma rays. 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. Schelrenchyma cells therefore cannot stretch, and they provide important structural support in mature stems after growth has ceased.
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Interestingly, schlerenchyma cells are dead at functional maturity. There are two types of sclerenchyma cells: fibers and sclereids. Fibers are long, slender cells; sclereids are smaller-sized. Sclereids give pears their gritty texture, and are also part of apple cores. We use sclerenchyma fibers to make linen and rope.
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:. Thus there is always a source of new cells at the tip of the shoot. The root tip has a similar population of meristematic cells that gives rise to all root tissues.
Both of these meristems are characterized by an indeterminate growth pattern: one that is not finite, but, in theory at least, could continue throughout the lifetime of the plant. Apical meristems are involved in several distinct developmental processes. The meristems are the location of cell proliferation and thus the source of all new cells in the shoot and root systems.
The regions below the meristems are the sites of active growth, as new shoot and root tissue rapidly expands. The shoot apical meristem plays a role in organogenesis, the formation of new leaves and axillary buds in a precise spatial pattern. In contrast, the root apical meristem is not involved in organogenesis; lateral roots are initiated by pericycle cells, which are themselves derived from the meristem, usually several centimeters away from the meristem.
The apical meristems also play a role in histogenesis by giving rise to cells that undergo distinct patterns of differentiation to form the specialized tissue types of the shoot and root. While the embryo initially gives rise to the precursors of dermal, ground, and vascular tissues protoderm, ground meristem, and procambium, respectively , these tissue precursors continue to be formed by the apical meristems and represent the first stages of cell and tissue differentiation.
Structure of root and shoot apical meristems. Growth is defined as an irreversible increase in mass that is typically associated with an increase in volume. Plant cell growth is associated with meristems and must be carefully regulated in order for organogensis and histogenesis to occur in the appropriate patterns. The plant regulates growth by regulating the extensibility of its cell walls. A cell that has nonextensible cells walls can take up some water, but eventually the physical pressure of the water inside the cell pressing out on the cell wall the turgor pressure prevents the entry of additional water and any further change in volume.
In contrast, a cell that has extensible cell walls can take up a substantial volume of water and thus increase in size. Turgor pressure that would otherwise prevent water entry momentarily decreases because the walls keep stretching. Typically cell growth occurs in small increments: 1 wall extensibility increases, reducing turgor pressure; 2 reduced turgor pressure allows water to enter the cell, increasing cell volume; 3 wall extensibility decreases, allowing the cell to build up turgor and preventing further water entry; and 4 the cell undergoes a cycle of synthesis of cytoplasmic and wall components, adding to the cell's mass.
30.1: The Plant Body
This cycle of incremental growth is repeated many times until the cell reaches its final size. The plant hormones auxin and gibberellin are produced in the vicinity of the apical meristems and usually act in concert to induce cell growth. Both hormones regulate wall extensibility, but carry out this function in different ways. Expansin cleaves the hydrogen bonds between two cell wall components: The cellulose microfibrils and the hemicellulose molecules that link adjacent cellulose microfibrils. Breakage of these bonds allows these structural wall components to reposition themselves farther apart, increasing wall extensibility.
Gibberellic acid, on the other hand, stimulates the activity of another cell wall enzyme called xyloglucan endotransglycosylase XET. Xyloglucans are a type of hemicellulose that is cleaved by the XET enzyme. Breakage of the The hormones auxin and gibberellin each promote cell expansion by loosening the bonds between adjacent cell wall molecules. Each hormone acts on a different molecular target. Cell division and cell growth are often tightly linked. When the rate of cell division is balanced by cell growth, as in the apical meristems, average cell size does not increase.
As the meristem grows away from earlier formed cells, the ratio of growth to division increases, resulting in overall cell enlargement. As the tissues mature further, cell division ceases completely, giving rise to zones of pure cell enlargement where most of the visible growth of the plant occurs. This relationship between division and growth, coupled with observations of the predictable planes of cell division during histogenesis, indicates that cell division is carefully regulated during plant development.
Molecules called cyclin-dependent kinases CDKs are key regulators of cell cycling including cell division in plants. CDKs are activated by association with a regulatory subunit called a cyclin and by phosphorylation and dephosphorylation events. The plant hormone cytokinin appears to regulate the cell cycle by interacting with the CDKs. Cytokinins enhance the synthesis of the cyclin subunits that are required for the cell to enter the deoxyribonucleic acid DNA synthesis phase of the cell cycle.
Cytokinins also enhance the CDK dephosphorylation step that is required for the cell to progress into mitosis. Both of these processes are inhibited by the hormone abscisic acid; thus a "developmental tug-of-war" occurs between a division-enhancing hormone and a division-suppressing hormone. The delicate balance between them determines the rate of cell division and this type of interaction is probably typical of the hormonal regulation of many aspects of plant development.
Differentiation is the process whereby cells, tissues, and organs become different from each other and from their precursors. The concept can be applied to organogenesis since cotyledons, foliage leaves, sepals , and petals may all develop from similar appearing precursors, the leaf primordia. As these organs mature, they become different from each other in size, shape, and the development of distinctive cell types. For instance, the epidermis tissue of petals is sharply differentiated from that of cotyledons, foliage leaves and sepals that are photosynthetic organs.
Correlated with a photosynthetic function, the epidermis of these organs is made up of flat, transparent cells that allow the penetration of light into internal tissues. Specialized guard cells that allow CO 2 to enter the leaf are also present. In contrast, the epidermal cells of petals contain brightly colored carotenoid or anthocyanin pigments. These cells also have a papillate shape that imparts a velvetlike sheen to the petal surface.
Since petals carry on minimal photosynthesis, they often lack guard cells. The process of differentiation is best understood on a cellular level. For instance, guard cells are highly specialized epidermal cells. Early in the development of a leaf, protodermal precursor cells undergo a distinctive pattern of cell divisions. At first the cell divisions are asymmetric, producing one large and one small derivative. The large derivative stops dividing and differentiates as an unspecialized epidermal cell, while the small derivative undergoes another asymmetric division.
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At an unknown stop signal, the small derivative undergoes a symmetric division, giving rise to two equal sized cells that become the guard cells. Unlike their plain neighbors, these cells develop a distinctive kidney shape, unevenly thickened cell walls, large, conspicuous chloroplasts, and finally form a pore the stomatal aperture between them.
Although plants share many features of development with animals such as apical-basal polarity, regulation of the balance between cell growth and cell division, formation of distinctive patterns of organs, cells and tissues, and differentiation, some aspects of development are unique to plants. Among these are:. Raven, Peter H. Evert, and Susan E. Biology of Plants , 6th ed. New York: W. Freeman and Company, Steeves, Taylor A. Patterns in Plant Development.
New York: Cambridge University Press, Wolpert, Lewis. Principles of Development.
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