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Plant Structure, Growth, and Development Overview: No Two Plants Are Alike The form of any plant is controlled by environmental and genetic factors. As a result, no two plants are identical. Plants have specialized organs that allow them to obtain water and nutrients from the soil and to absorb water and CO2 from the air. Plant species have adaptive features that benefit them in their specific environments. Can you think of some adaptations of plants and their function? Plants Structure Plants, like multicellular animals, have organs that are composed of different tissues, and tissues are composed of different cell types. o o A tissue is a group of cells with a common structure and function. An organ consists of several types of tissues that work together to carry out particular functions. Plant organs include roots, stems, leaves and flowers. Plant Tissues: Each organ of a plant has three tissue systems that are continuous throughout the plant: dermal, vascular, and ground. 1. Dermal tissue is the outer covering. In nonwoody plants, it is a single layer of tightly packed cells, or epidermis, that covers and protects all young parts of the plant. Root hairs are extensions of epidermal cells near the tips of the roots. The epidermis of leaves and most stems secretes a waxy coating, the cuticle, which helps the aerial parts of the plant retain water. 2. Vascular tissue, continuous throughout the plant, is involved in the transport of materials between roots and shoots. There are two main types of vascular tissue 1. Xylem moves water and dissolved minerals upward from roots into the shoots. 2. Phloem transports food made in mature leaves to the roots; to nonphotosynthetic parts of the shoot system; and to sites of growth, such as developing leaves and fruits. 3. Ground tissue is tissue that is not dermal tissue or vascular tissue. o Functions of ground tissue include photosynthesis, storage, and support. Plant Cell Types: Plant tissues are composed of three basic cell types: parenchyma, collenchyma, and sclerenchyma. After formation, many plant cells become differentiated, meaning that each cell grows and develops structural adaptations that allow it to do a specific job in the cell. The major types of differentiated plant cells include: parenchyma, collenchyma, sclerenchyma, water-conducting cells of the xylem and sugar-conducting cells of the phloem. 1. Parenchyma cells: * Mature parenchyma cells have primary walls that are relatively thin and flexible, and most lack secondary walls. * They usually have a large central vacuole. * Parenchyma cells are often depicted as “typical” plant cells because they generally are the least specialized. There are exceptions to this generalization, for example, the highly specialized sieve-tubes of the phloem are parenchyma cells. * Parenchyma cells perform most of the metabolic functions of the plant - parenchyma cells in the leaf photosynthesize - parenchyma cells in stems and roots store starch - the fleshy tissue of most fruit is parenchyma tissue * Most parenchyma cells retain the ability to divide and differentiate into other cell types under special conditions, such as the repair and replacement of organs after injury to the plant. In the laboratory, it is possible to regenerate an entire plant from a single parenchyma cell. 2. Collenchyma cells have thicker primary walls than parenchyma cells, though the walls are unevenly thickened. They are often grouped into strands or cylinders, and are located just below the epidermis and provide support. Mature collenchyma cells are living and flexible and elongate with the stems and leaves they support. 3. Sclerenchyma cells have thick secondary walls usually strengthened by lignin and function as supporting elements of the plant. They are much more rigid than collenchyma cells. Unlike parenchyma and collenchyma cells, they cannot elongate, so sclerenchyma cells occur in plant regions that have stopped lengthening. Many sclerenchyma cells are so specialized that they are dead at functional maturity, having produced the secondary walls before the protoplast died. Two types of sclerenchyma cells, fibers and sclereids, are specialized entirely for support. Fibers are long, slender, and tapered, and usually occur in groups. Examples include hemp used to make rope and flax used to make linen cloth Sclereids are irregular in shape and shorter than fibers. They have very thick, lignified secondary walls. Sclereids impart hardness to nutshells and seed coats and the gritty texture to pear fruits. 4. Xylem: Xylem is composed of various types of cells: parenchyma, fibers, and special water-conducting cells of xylem called tracheids and vessel elements. Tracheids and vessel elements are elongated cells that are dead at functional maturity. The thickened secondary cell walls remain as a nonliving conduit through which water can flow. Tracheids are long, thin cells with tapered ends Water moves from cell to cell mainly through pits: thin-walled areas between cells through which water and dissolved substances can flow. Because their secondary walls are hardened with lignin, tracheids function in support as well as transport. Vessel elements are generally wider, shorter, thinner walled, and less tapered than tracheids. Xylem tracheids and vessels elements Phloem sieve tubes and Companion cells 5. Phloem moves sucrose, other organic compounds, and some mineral ions move through tubes formed by chains of cells called sieve-tube members. Phloem tissue includes parenchyma, schlerenchyma, sieve tubes and companion cells. Phloem cells are alive at functional maturity, although a sieve-tube cell lacks a nucleus, ribosomes, and a distinct vacuole. Adjoining end walls, contain sieve plates which have pores that facilitate the flow of fluid between cells. Each sieve-tube member has a nonconducting nucleated companion cell, which is connected to the sieve-tube member by numerous plasmodesmata. The nucleus and ribosomes of the companion cell serve both that cell and the adjacent sieve-tube member. In some plants, companion cells actively help load sugar into the sieve-tube members, which transport the sugars to other parts of the plant. Vascular plants have three basic organs: roots, stems, and leaves. The basic morphology of vascular plants reflects their evolutionary history as terrestrial organisms that inhabit and draw resources from two very different environments: They obtain water and minerals from the soil. They obtain CO2 and light above ground. To obtain the resources they need, vascular plants have evolved two systems that must work together: 1. 2. a subterranean root system that absorbs water and minerals for the entire plant an aerial shoot system of stems and leaves that photosynthesize to provide sugar for the entire plant. ROOTS: A root is an organ that functions to: a. anchors a vascular plant in the soil, b. absorbs minerals and water, and stores food. c. Store food (in many plants) Absorption of water and minerals occurs near the root tips, where vast numbers of tiny root hairs enormously increase the surface area. Root hairs are extensions of individual epidermal cells on the root surface. Absorption of water and minerals is also increased by mutualistic relationships between plant roots and fungi (Mycorrhizae) or by plant roots and bacteria (Ex Nitrogen fixing bacteria) Most eudicots and gymnosperms have a taproot system, consisting of one large vertical root (the taproot) that produces many small lateral, or branch, roots. taproots often store food that supports flowering and fruit production later. Seedless vascular plants and most monocots, including grasses, have fibrous root systems consisting of a mat of thin roots that spread out below the soil surface. A fibrous root system is usually shallower than a taproot system. Grass roots are concentrated in the upper few centimeters of soil. As a result, grasses make excellent ground cover for preventing erosion. Anatomy of a root: Root Cap: The tip of the root is covered by a cap that is shaped like a thimble. The outer cells of the root cap are continuously being worn away and new cells are added to the inner portion. As these cells disintegrate they form a strong protective cover. Zone of Cell Division: A zone of active cell division (called an apical meristem) which rapidly forms new cells which differentiate later to form more specialized root tissues. The cells of this region also replace the cells rubbed-off from the root cap Zone of Elongation: Cells in this zone undergo rapid growth in length and cause the root to elongate and penetrate deeper into the soil. Epidermis: of the root is dermal tissue and is made up of thin walled parenchyma cells with no cuticle. Its function is to protect the root. Cortex: the cortex is a fairly wide region of ground tissue that is active in the uptake of water and minerals and also functions to store starch and other molecules. Stele: the bundle of vascular tissue (xylem and phloem) Endodermis: a cylinder of cells one cell thick that forms a boundary between the cortex and the stele It contains the casparian strip, a cell wall layer containing suberin and sometimes lignin on the endodermis cells that is waterproof and prevents water entering the vascular tissue without first moving through the plasma membrane and into a cell. Pericycle: a layer of cells found just inside of the endodermis, it may become meristematic and result in the formation of lateral roots Root anatomy of eudicots: Xylem and phloem in groups of 6-8 Xylem forms and “X” shape in stele (center) Pith is absent or small (Pith is found in the very central region of some roots and stems, and is made up of parenchyma cells). Xylem elements are angular or polygonal in transverse section. Root anatomy in monocots: Xylem and phloem in many alternating bundles forming circle around the root. Pith is large and well developed Modified Roots: Some arise from roots while adventitious roots arise Above ground from stems or even from leaves. Some modified roots provide additional support and anchorage. Others store water and nutrients or absorb oxygen or water from the air Storage roots (sweet potato) Air roots (mangrove) enable plants to get extra oxygen in water logged soil Mangrove tree STEMS: A stem is an organ that raises or separates leaves, exposing them to sunlight. A stem consists of : alternating nodes, the points at which leaves are attached, internodes, the stem segments between nodes. At the angle formed by each leaf and the stem is an axillary bud with the potential to form a lateral shoot or branch. Growth of a young shoot is usually concentrated at its apex, (tip) where there is a apical meristem , or an area of sustained cell division. Cells differentiate to form vascular tissue, leaves, stem cells. As the cells mature, they elongate and cause the stem to elongate. The terminal bud produces hormones that travel down the stem and inhibiting the growth of axillary buds, a phenomenon called apical dominance. By concentrating resources on growing taller, apical dominance is an evolutionary adaptation that increases the plant’s exposure to light. In the absence of a terminal bud, the axillary buds break dominance and give rise to a vegetative branch complete with its own terminal bud, leaves, and axillary buds. By pruning or pinching the apical meristem (bud) off of a plant, you can encourage the axillary buds to grow and cause branching. Monocot and herbaceous Eudicot (Dicot) stems vary in the arrangement of their vascular tissue. Pith In eudicot stems, ground tissue is divided into pith , located inside the vascular tissue, and cortex, found outside the vascular tissue Modified shoots with diverse functions have evolved in many plants. Stolons, such as the “runners” of strawberry plants, are horizontal stems that grow on the surface and enable a plant to reproduce asexually as plantlets form at tips of each runner. Rhizomes, Sturdy, horizontal, underground stems called rhizomes anchor large monocots such as palms and bamboo. Another example is ginger. Tubers, including potatoes, are the swollen ends of rhizomes specialized for food storage. Bulbs, such as onions, are vertical, underground shoots consisting mostly of the swollen bases of leaves that store food. Tuber Bamboo rhizome Bulb LEAVES Leaves are the main photosynthetic organs of most plants, although green stems are also photosynthetic. o o o o While leaves vary extensively in form, they generally consist of a flattened blade and a stalk, called the petiole, which joins the leaf to a stem node. Grasses and other monocots lack petioles. In these plants, the base of the leaf forms a sheath that envelops the stem. Most monocots have parallel major veins (bundles of xylem & phloem) that run the length of the blade Eudicot leaves have a multibranched network of major veins. Most leaves are specialized for photosynthesis. Some plants have leaves that have become adapted for other functions - tendrils that cling to supports (peas, cucumbers) - spines of cacti - leaves modified for water storage (jade plant, sedum) Structure of a leaf: Cuticle: waxy layer on top and bottom epidermis. Minimizes water loss. Vein: a bundle of xylem and phloem. Palisades cells: tightly packed vertically arranged cells located under the upper epidermis. They’re main function is photosynthesis. Spongy mesophyll: irregular shaped cells interspersed with many air spaces that allow for gas (CO2) exchange. They usually contain a few chloroplasts and carry out photosynthesis. Stomata are tiny pores in the epidermis (typically the lower epidermis). They consist of two specialized guard cells that actively intake K+ ions, causing an intake of water that swells (opens) the pore. The guard cells “deflate” or close when they loose water, thus closing the opening in the leaf. Stomata function to control the exchange of gases (CO2 and water vapor) in the leaf. Determinate vs Indeterminate Growth A major difference between plants and most animals is that plant growth is not limited to an embryonic period. Most plants demonstrate indeterminate growth, growing as long as the plant lives. In contrast, most animals, some plants, and certain plant organs, such as flowers and leaves, undergo determinate growth, ceasing to grow after they reach a certain size. Annual – Biennial - Perennial Annual plants complete their life cycle—from germination through flowering and seed production to death—in a single year or less. o Many wildflowers and important food crops, such as cereals and legumes, are annuals. A biennial plant’s life spans two years. Biennials often grow vegetatively the first year, the flower the second year, often with a cold season in-between. A perennial is a plants such as trees, shrubs, and some grasses that live many years. Perennials do not usually die from old age, but from an infection or some environmental trauma. A perennial is capable of indeterminate growth because it has perpetually embryonic tissues called meristems in its regions of growth. These cells divide to generate additional cells, some of which remain in the meristematic region, while others become specialized and are incorporated into the tissues and organs of the growing plant. Primary Growth is the increase in shoot or root length through cell division at apical meristems , located at the tips of roots and in the buds of shoots, supply cells for the plant to grow in length. In herbaceous plants, primary growth produces almost all of the plant body. Herbaceous plants die back every year when the weather gets cold. Herbaceous plants do not have a woody stem that stays above ground when the leaves are gone. Secondary Growth in perennials: Woody plants also show secondary growth, progressive thickening of roots and shoots where primary growth has ceased. Secondary growth is produced by lateral meristems, cylinders of dividing cells that extend along the length of roots and shoots. 1. The vascular cambium adds layers of vascular tissue called secondary xylem and phloem. 2. The cork cambium replaces the epidermis with thicker, tougher periderm.(bark) The vascular cambium is located just outside the primary xylem and to the interior of the primary phloem. The cells of the vascular cambium divide and form secondary xylem (tracheids and vessel elements) to the inside and secondary phloem (sieve elements and companion cells) to the outside. The thickening of the stem that occurs in secondary growth is due to the formation of secondary phloem and secondary xylem by the vascular cambium, plus the action of cork cambium which forms the bark. The cells of the secondary xylem contain lignin, which provides hardiness and strength. In woody plants, cork cambium is the outermost lateral meristem. It produces cork cells (bark) containing a waxy substance known as suberin that can repel water. The bark protects the plant against physical damage and helps reduce water loss. The cork cambium also produces a layer of cells known as phelloderm, which grows inward from the cambium. The cork cambium, cork cells, and phelloderm are collectively termed the periderm. The periderm substitutes for the epidermis in mature plants. Growth Rings: The activity of the vascular cambium gives rise to annual growth rings. During the spring growing season, cells of the secondary xylem have a large internal diameter; their primary cell walls are not extensively thickened. This is known as early wood, or spring wood. During the fall season, the secondary xylem develops thickened cell walls, forming late wood, or autumn wood, which is denser than early wood. This alternation of early and late wood results in the formation of an annual ring, which can be seen as a circular ring in the cross section of the stem . An examination of the number of annual rings and their nature (such as their size and cell wall thickness) can reveal the age of the tree and the prevailing climatic conditions during each season. Angiosperms produce reproductive structures called flowers and fruit. Flowers: shoots that have become specialized for sexual reproduction. The flower structure has up to four rings of modified leaves: Starting at the base (stem end of the flower) these rings of modified leaves are: 1. Sepals: usually green, they are the outer layer of the bud and encase the flower before it opens. 2. Petals: generally brightly colored they function to attract pollinators. - wind pollinated flowers usually lack colorful parts The sepals and petals are sterile, meaning they are not involved with the production of eggs and sperm. 3. Stamen: located within the petals, stamen produce microspores that develop into pollen grains that contain the male gametophyte. - have a stalk called the filament - terminal end contains a sac called the anther where the pollen is produced. 4. Carpel – located within the petals, the carpel produces a single or multiple female gametophytes. - Stigma – the sticky tip that receives the pollen - Style – connects the stigma to the ovary - Ovary – located at the base of the carpel, it contains one or more ovules (containing the female gametophytes) If fertilized, the ovule develops into a seed. Fruit usually consists of a mature ovary. As the seed develops, the walls of the ovary thicken. The fruit protects the seeds and aids in the dispersal of the seeds. Fruits may be fleshy (grape, tomato) or dry (bean, nut, grain).