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Stems and Roots Chps 9 and 10 Chp 9 Vocabulary • Lignin • Secondary xylem • Tracheids • Vessel elements • Secondary phloem • Primary xylem • Cork • Primary phloem • Periderm • Vascular bundles • Suberin • Sieve elements • Pressure (mass) flow • Companion cells • Vascular cambium • Symplastic • Cork cambium loading Stems are Fundamental Plant Organs • Vascular plants are those plants that have a conducting system composed of vascular tissue (xylem and phloem). • Stems are indispensable organs for most plants. All other organs (leaves, buds, roots) are attached to stems. Stems enable plants to increase their height or length, mass, and surface by the activity of apical meristems. • Plant stems are usually branched, which allows increase in mass and the amount of surface available for attachment of leaves and reproductive structures. The more leaves on a stem, the greater the amount of sunlight they can harvest in photosynthesis. • Stems transport water and minerals collects by roots from the soil to the leaves where these materials are needed for photosynthesis (xylem), and conduct sugars produced in the leaves to roots and any other places where sugar fuel is needed (phloem) • Xylem is defined by the presence of the tough, waterproofing compound lignin on walls of specialized cells – tracheids and vessel elements. Lignin also provides support to vascular tissue and thus the plant. Structure and Function of Stems • In herbaceous (nonwoody) stems and the young stems of woody plants, xylem and phloem tissues differentiate from precursor tissue (procambium) formed by the apical meristem. • Mature conducting tissues formed in this way are known as primary xylem and primary phloem. These primary conducting tissues are located near each other within elongate vascular bundles. • The vascular tissue in a plant is interconnected – extending from the roots, through thte stem, into branches and leaves and other organs like the water pipes in your house. Phloem Tissues • Phloem tissues include pipeline components known as sieve elements, which may consist of sieve cells or sieve tube members end to end. • Sieve elements possess pore-containing end walls known as sieve plates. Their perforations develop by expansion of plasmodesmata. Pores in the end walls of sieve elements allow phloem sap – a watery solution of sugars and other organic molecules – to move freely from one cell to the next. • Phloem sieve elements are alive at maturity, but the nucleus and some other cell components are degraded during sieve element development and are thus absent from mature cells. In order to function, sieve elements require the help of the adjacent companion cells, which have nuclei and provide materials to the sieve cells via plasmodesmata. • Ex. When a plant is cut or wounded, P protein (phloem protein) masses along the sieve plate of sieve elements, forming a “slime plug”. Such plugs function to reduce the loss of phloem sap, much as clots reduce blood loss from the vascular system of animals. • Another wound response is deposition of the carbohydrate callose, which also helps to plug phloem sap leaks. Phloem Conducts Sugars • Phloem provides plants with a long-distance transport system. The direction of transport in phloem is from the source of organic molecules to sites, known as “sinks”, where molecules are utilized like roots, flowers, etc. • Direction is “source to sink” • In some plants, sugars are loaded from the cells producing them either directly into sieve elements or indirectly, via companion cells through plasmodesmata. This process is known as symplastic loading. • Other plants have apoplastic loading of phloem, which occurs from intercelular spaces. This means that it will require active transport through the cell membrane. • The force that moves organic compounds within the phloem is known as pressure flow or mass flow and is based on osmosis and cell water potential. Water Moves bc of Transpiration • With a few exceptions, plants obtain their water and minerals from the soil and move these materials via the root’s xylem into the stem. The stem’s xylem’s main function is to transport water and minerals to other organs. • Water moves in the xylem as the result of transpiration, the evaporation of water from plant surfaces like the stomata. A stream of water rises through the plant as each water molecule lost from a cell at the surface is replaced by another from inside the cell, which in turn exerts an attractive force on nearby water molecules, causing the water to rise. • Xylem will help new leaves and flower buds grow by placing sugar in a watery solution to flow up the tree (days are warm, nights are cold in spring). Maple trees are good sources of such xylem sap and are cultivated in large plantations called sugar bushes. With care, 150 L can be tapped per year without harming the tree. Wood and Bark • Many plants produce no wood or bark, but woody plants produce wood tissue and bark by the action of two meristems (vascular cambium and cork cambium). • The girth increase of a tree trunk is known as secondary growth. The cambiums are secondary meristems. Vascular Cambium • Mature vascular cambium takes the form of a cylinder. It produces lignin-rich secondary xylem tissue on the inside (wood) and secondary phloem on the outside (inner bark). • Addition of a thick cylinder of wood requires that the circumference of the vascular cambium must increase, necessitating the addition of new vascular cambium cells by cell division. • Ray initials produce ray parenchyma cells and ray tracheids (together form vascular rays). Rays store things and transport food laterally across the stem. • During each growing season, the vascular cambium produces new cylinders of secondary xylem, adding new wood and growth rings. Rings from previous years may still transport water, but really old rings (toward the center) can become clogged by tyloses from neighboring parenchyma cells. • Innermost wood is heartwood: full of decay-resistant chemicals, good for furniture • Phloem exists towards the outside, in the inner bark layer. They are susceptible to bark damage because it would cut off its food supply (girdling or ringing a tree will kill it). Cork Cambiumbegin to enlarge, the • As young woody stems delicate epidermis eventually ruptures and its protective role is replaced by cork. • Cork is produced to the outside of a secondary meristem called the cork cambium. • Together, the cork, cork cambium and parenchyma cells make up the periderm. When periderm become worn out, they will be replaced by a new periderm on the inside. Eventually the old periderm will create outer bark. The outer bark is dead whereas inner bark is still living. • Cork cell walls have layers of lignin and suberin. Suberin helps prevent microbial attack and also waterproofs the stem’s surface. • Lenticels are slightly raised patches of various shape that interrupt the bark’s cork layer to allow gas exchange for the inner stem tissues. Human Uses for Stems • Paper: Paper started as papyrus, which was made from the stems of the papyrus plant. Papyrus also made rafts, sails, cloth, and cord. To make paper, the Egyptians peeled the outer layers of papyrus stemss off, exposing the pith. The pith was sliced into thin strips and laid across one another. Workers then pounded the layers making starch release from the cells and thus gluing the strips together and then dried in the sun. Today, most paper is made from wood pulp. Genetic engineers are working on making trees with more cellulose and less lignin for paper. Lignin byproducts of the pulp are toxic and can threaten water supply. • Cork: The cork oak’s cork layer is several inches thick. It can be stripped without hurting the tree. Cork is able to float and is used as insulation, floor covering, shoe soles and bottle stoppers. • Bamboo: used for housing in some areas. UK is working on creating earthquake-proof housing with bamboo. • Wood: construction material, fuel, paper, furniture. Known for strength and beauty and makes up more than 1% of the world’s total economy. Species like redwood and white oak are desired for ship building. Basswood, yellow birch, and black cherry are valued for making musical instruments because their structure lends to a beautiful tone. Chapter 10 Vocabulary • Embryonic root • Storage root •Feeder root • Prop root •Root hairs • Aerial root •Gravitropism •Mucigel • Buttress root •Pericycle • Lenticel •Micorrhizal fungi • Epiphytic plants •nitrogen-fixing bacteria • Taproot system Roots Play a Variety of Roles • When seeds germinate, the first plant organ to emerge is the embryonic root, the radicle,– and a primary root is soon present on young plants. • Shoot development depends on enlargement of cells by water uptake. And photosynthesis requires water to serve as the necessary electron donor. Both of these processes are highly dependent on an early water and mineral supply. • Bryophytes do not have roots. Moss and bladderworts are examples. Because their leaves are so thin, they can directly absorb water and minerals from their very wet environment. • Some roots store carbohydrates during the first year of growth of biennials. Carrots, sugar beets, parsnips, and rutabagas are biennials grown for their food-roots. Roots are Hormone and Secondary Compound Sites • Roots produce the plant hormones cytokinins and gibberellins, which are transported in the xylem to the shoot, where they influence growth and development. • Roots are also a site for producing protective secondary compounds. Ex. Nicotine is made in the roots of tobacco plants and moved to the leaves to act as a poison that helps prevent herbivore attack. • The roots of an African tree has long been known to produce a yellow substance used by healers to treat syphilis and leprosy (both caused by bacteria). Recent studies showed that the yellow compound (identified as a terpene) does in fact kill bacteria and fungi. Root Support • If you look closely at the base of corn plants, you may notice prop roots, growing from the stem into the soil. These specialized roots help the tall corn plants stay upright even though they lack woody tissue. • Some tropical trees grow in thin soil and use buttress roots to help keep from falling over on a windy day. • Aerial roots form from a stem and form massive columns to support the heavy branches. Specialized Roots • Pneumatophores (“breath bearers”) are specialized roots of some types of mangrove trees. They grow upward into the air, absorb oxygen rich air via surface openings – lenticels. • When the tide is up, the lenticels are protected by waterproofing substances. When the tide goes back down, air is sucked into the lenticels. • Some herbaceous plants like dandelions have contractile roots which shorted by collapsing their cells. This allows the root to pull deeper into the ground where it’s warm to survive changing early spring weather. • Parasitic plants like the dodder obtain water, minerals, etc from host plants by producing rootlike organs that penetrate the host’s stem and tap into the host’s vascular system. • Epiphytic plants grow non-parasitically on other plants and have specialized roots Their roots are aerial and are photosynthetic. Ants often form an association with such plants and provide nitrogen to the plant with their waste. Types of Underground Root Systems • Plant roots differ in their external form. These differences result from variations in the fate of a seedling’s primary root. For example, in gymnosperms and eudicot angiosperms, the primary root generates a taproot system – single main root from which many branches emerge. • In grasses and other monocots, the primary root lives for a short time and is replaced by a system of roots that develop from the bottom of the plant’s stem. Roots from a stem are called adventitious roots. Many adventitious make up a root system. • If no single root is most prominent, then we say it’s a fibrous system (many branched roots). These are usually shallower in the ground than a taproot. • Feeder roots, produced by both taproot and fibrous root systems are fine (<2mm in diameter) peripheral root that are most active in absorbing water and minerals from the soil. Feeder roots have limited lifespans and are continuously replaced. • Knowledge of feeder roots is helpful in landscaping. When transplanting a plant, make sure you know where the roots end so you do not risk cutting them when digging the plant up, otherwise the plant may not be able to obtain nutrients from the injured roots and not survive. External Root Root Structure and Function • Feeder roots are young branch roots. Soil texture influences root branching. Plants that must grown through hard, dry soil have fewer branch roots than those growing in moist, loose soil. • Branch roots and the main root axis are covered by an epidermis, which is sometimes covered by a cuticle. • A region closer to the root tip is fuzzy with countless root hairs – fingerlike extensions from some epidermal cells. For most roots, these hairs are the main location of water and mineral absorption, and root hairs are a major site of uptake selectivity, the ability of plant roots to discriminate between useful and harmful soil minerals. • At the cone-shaped tip, there is a root apical meristem (RAM). This region of meristematic cells, which divide rapidly, increasing the number of cells in the main portion of the root. • Protecting the RAM is a root cap, whose cells are also generated by the apical meristem. Cells in the center of the root cap contain starch-rich plastids, amyloplasts. Some experts think that amyloplasts operate as gravity sensors since they are heavy enough to fall as the root grows, thus signaling the downward growth path normal to most root cells. • Other experts believe there are different mechanisms for gravitropism, a root’s growth response to gravity. Plant biologists do not fully understand why plants know which way is down. • Root cap cells slough off the root tip a few days after being created, so they must continually be replaced. These dispersal cells, known as root border cells, do not then just die, but apparently help modify the external root environment in ways that prevent attack by microbes and tiny soil worms. • The tips of roots are embedded in a blanket of mucigel, a gluey substance secreted from the Golgi apparatus of root tip epidermal cells. Mucigel lubricates the root and helps in water and mineral absorption and creates a favorable environment for beneficial microbes. Root Mineral Absorption • Root xylem obtains minerals and water in one of two ways – Water and minerals are selectively taken up by root hairs and transmitted via plasmodesmata – Water and minerals that penetrate root tissues within intercellular spaces and cell walls are selectively absorbed at the cell membrane of nonsuberized surfaces of endodermal cells and released on the other side. • Mineral passage from root hairs through the cortex and endodermis via plasmodemata is known as symplastic transport. This allows beneficial minerals to be absorbed and harmful ones to be excluded. • When minerals dissolved in water diffuse from the root’s environment into epidermal cell walls, then through walls of cortical cells to the endodermis, such movement is known as apoplastic transport. In apoplastic transport, harmful minerals are unable to be excluded which could cause the plant to be injured. Root Hairs Have Selective Absorption • Epidermal root hairs and cells of the endodermis filter out mineral content of water. • Many metal ions (iron, copper, manganese, and magnesium) are needed by plants for the proper functioning of enzymes and other complex molecules in plant cells. Magnesium is used in chlorophyll and iron is an electron carrier in photosynthesis and respiration. • Aluminum is abundant in soil but toxic to plants. It can bind to things like proteins and nucleotides causing disruption in membrane function. The first symptom of aluminum toxicity in plants is that roots stop elongating within 5 minutes of exposure. • Aluminum toxicity is a major limitation in growing crops. Its more prominent in acidic soils in high industry areas. • Acid rain can turn soil acidic. When soil gets below a pH of 5, positively charged metals stick to the soil releasing aluminum ions that are then dissolved in the soil water and available for absorption. Plants can bind the aluminum by releasing organic acid, but the best way to avoid it, • Phosphate is needed to construct phospholipid membranes, ATP, and DNA/RNA. Phosphate is one of the most important components for plants. • Phosphate forms strong chemical bonds with iron and aluminum oxide minerals in solid, reducing its availability. The organic acids mentioned earlier can help dissolve the aluminum, freeing phosphate. • Plant root cell membranes contain transporter proteins whose shapes enable them to bind even small amounts of soil phosphate and move it into the cell. When soil phosphate is low, root cells increase the number of phosphate transporter proteins Roots Need Food and Oxygen • Plant root cells are efficient at mineral absorption, but they use a lot of ATP. ATP is also required for cell division at the root tip. • The most efficient mode of ATP production is aerobic respiration (uses O2). Roots cannot photosynthesis, so the phloem must bring them sugar sap. • Roots of a plant are called heterotrophic because they (like animals) must consume their food (don’t make it). • Root produced carbon dioxide dissolves in soil water to produce carbonic acid which will cause weathering of the soil. This is another way plants help reduce carbon dioxide. Beneficial Microbes • Beneficial microorganisms can form symbiotic relationships with plant roots. • These include mycorrhizal fungi and nitrogen-fixing bacteria, which live within roots of legumes and some other plants. • These microbes help plants obtain the large amouts of minerals needed for growth (this growth would otherwise be limited). This allows for a better competing plant and a higher crop yield. • Mycorrhizal fungi are important in providing phosphate. Almost all vascular plants have mycorrhizal fungi. • Nitrogen-fixing bacteria supply the nitrogen compounds required by plants to produce amino acids and proteins. Legumes have much closer associations with such bacteria, producing special root nodules whose tissues harbor nitrogen-fixing bacterial partners. • Legume-bacterial relationships begin with a chemical conversation. • Legume roots secrete flavonoids into the soil (secondary compound) • Legume-root flavonoids signal to soil N-fixing bacteria, which respond to the flavonoid signal by secreting small organic molecules into the soil. • Legume-root epidermal cell membranes contain receptor molecules that recognize and bind molecules excreted from specific bacteria. These bacterial compounds cause the root hairs to curl within special root cells. • Root nodules contain both infected and uninfected cells and mature nodules possess vascular tissue that connects with the root vascular system that distributes compounds containing nitrogen throughout the plant. • The bacteria are plant specific due to the secondary compounds released.
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