B120 - W24 - Stud - T9 - Plant Structure and Function.pdf

Plant Structure and Function Vinicius Azevedo Non-vascular Plants X Vascular Plants Terrestrial members of the kingdom Plantae (plants) are divided into two broad groups: Non-vascular Plants Vascular Plants ü Include the Bryophytes üLack of roots, stems and leaves üWater is taken by direct absorptio...

Plant Structure and Function Vinicius Azevedo Non-vascular Plants X Vascular Plants Terrestrial members of the kingdom Plantae (plants) are divided into two broad groups: Non-vascular Plants Vascular Plants ü Include the Bryophytes üLack of roots, stems and leaves üWater is taken by direct absorption through their cell walls and moves from cell to cell via osmosis (water under the soil is not reached) üCannot grow tall Hornwort Liverwort üInclude Ferns, Gymnosperms (conifers) and Angiosperms (flowering plants) üHave roots, stems and leaves üHave vascular tissue (special cells for transport of water and nutrients, which run continuously through roots, stems and leaves) üCan grow tall Moss Fern Gymnosperm Angiosperm Seedless Vascular Plans: Ferns Have roots, stems and leaves Have vascular tissues (xylem and phloem) Do not have seeds Do not have flowers Depend on the water for reproduction Holly Fern Japanese Painted Fern Boston Fern Australian Tree Fern Vascular See Plants: Gymnosperms and Angiosperms Gymnosperms Angiosperms ü Non-flowering plants ü Flowering plants ü Plant seeds are not enclosed in an ovule, also known as “naked seeds” (i.e. pine cone) ü Mature seed is surrounded by ovule (usually in a fruit) ü Leaves are modified into needles that stay green throughout the year ü Wind pollination is most common (pollination is the process by which pollen is transferred from the anther [male part] to the stigma [female part] of the plant, thereby enabling fertilization and reproduction) ü Leaves are flat, broad and usually change in colour and die every autumn (i.e. oaks and maples) ü Insect pollination is most common ü More abundant than gymnosperms ü Divided into eudicots and monocots, their differences will be covered in this lecture. The Plant Body Like multicellular animals, plants have organs composed of different tissues, which in turn are composed of cells Three basic organs evolved: roots, stems, and leaves They are organized into a root system and a shoot system Roots rely on sugar produced by photosynthesis in the shoot system Shoots rely on water and minerals absorbed by the root system Roots Roots are multicellular organs with important functions: ü anchoring the plant ü absorbing minerals and water ü storing organic nutrients The roots of eudicot plants form a taproot system, which consists of one main vertical root that gives rise to some large lateral roots, or branch roots The roots of monocot plants form a fibrous root system, which is characterized by many thin lateral roots with no main root In most plants, absorption of water and minerals occurs near the root hairs, where vast numbers of tiny root hairs increase the surface area Root hairs often form on fine roots and improve water absorption by increasing root surface area and by improving contact with the soil Roots: Water and Minerals Absorption After water and minerals enter the root hair, they travel through either the: ü Symplastic route: via the continuous connection of cytosol between cells through plasmodesmata ü Apoplastic route: via the cell walls and intercellular spaces The endodermis (also known as the root’s border guard) is the innermost layer of cells in the root cortex… this is the last checkpoint for selective passage of minerals from the cortex into the vascular cylinder (stele) The waxy Casparian strip of endodermal wall blocks apoplastic transfer of minerals from the cortex to stele (this forces polar molecules taking the apoplastic route to enter endodermal cells using the symplastic route) Roots: Eudicots X Monocots In eudicots, the xylem forms a cross or star while the phloem is found between the arms of the star/cross In most monocot, xylem and phloem form a ring with the core comprising of parenchyma cells (referred to as the pith) Shoot System: Stems Stems are plant organs consisting of nodes and internodes ü Nodes = the points at which leaves are attached ü Internodes = the stem segments between nodes An axillary bud, or lateral bud, is a embryonic structure found at the junction of the stem and petiole that has the potential to form a lateral shoot, or branch Apical bud, or terminal bud, is an embryonic structure located near the shoot tip and causes elongation of a young shoot Stems: Eudicot X Monocot In most eudicots, the vascular tissue consists of vascular bundles (xylem and phloem group together) that are arranged in a ring Pith is present in cross section In most monocot, the vascular bundles are scattered randomly throughout the ground tissue Pith NOT present in cross section (but not in monocot stem) Shoot System: Leaf Structures Leaves are the main photosynthetic plant organs They generally consist of a flattened blade and a stalk called the petiole, which joins the leaf to a node of the stem Monocots and eudicots differ in the arrangement of veins, the vascular tissue of leaves: – Most monocots have parallel veins – Most eudicots have branching veins Monocot parallel venation Eudicot net venation Shoot System: Leaf Shapes Simple – leaf has a single undivided blade (i.e. pear) Compound – leaf blade consists of multiple leaflets (i.e. roses) ü A leaflet has no axillary bud at its base Doubly-compound – each leaflet is divided into smaller leaflets (i.e. honey locust) Advantages of compound or doubly-compound: ‒ Enable leaves to withstand strong wind with less tearing ‒ Confine some pathogens (disease-causing organisms and viruses) that invade them to a single leaflet rather than allowing them to spread to the entire leaf There is a difference between monocots and eudicots in which the leaves are orientated off the stem ü Eudicot leaves generally come off the stem horizontally ü Monocot leaves generally come off the stem vertically Monocot Eudicot Leaf: Stomata The epidermis in leaves is interrupted by stomata, which allow CO2 exchange between the air and the photosynthetic cells in a leaf Each stomatal pore is flanked by two guard cells, which regulate its opening and closing ü In most eudicot leaves, stomata are found in greater numbers on the leaf underside compared to the top ü In most monocot leaves, stomata are found in approximately the same numbers on both sides The ground tissue in a leaf, called mesophyll, is sandwiched between the upper and lower epidermis ü Eudicot leaves have palisade mesophyll in the upper part of the leaf while the lower part contains loosely arranged spongy mesophyll, where gas exchange occurs ü Monocot leaves only have spongy mesophyll Eudicot leaf cross-section Palisade mesophyll Spongy mesophyll Vascular bundle of vein Monocot leaf cross-section Stoma with guard cells Vascular bundle of vein Spongy mesophyll Categories of plant tissue There are four basic categories of plant tissue: 1. 2. 3. 4. Dermal Vascular Ground Meristematic Apical meristem Dermal In non-woody plants, the dermal tissue system consists of the epidermis (a single layer of cells that covers and protects the young parts of the plant). A waxy coating called the cuticle helps prevent water loss from the epidermis (this is for shoot system only…NOT root system). In woody plants, protective tissues called periderm replaces the epidermis in older regions of stems and roots (periderm is part of the bark). Besides epidermis, cuticle and periderm, two other examples of dermal tissue are guard cells (of stomata) and root hairs. Vascular The vascular tissue system carries out long-distance transport of materials between roots and shoots Xylem conveys water and dissolved minerals upward from roots into the shoots ü Uni-directional movement of water ü Water-conducting cells (tracheids and vessel elements) Phloem transports organic nutrients from where they are made to where they are needed (can move from roots to shoots or from shoots to roots) ü Bi-directional movement ü Sugar-conducting cells (sieve-tube elements and companion cells) xylem The two xylem cell types, the tracheids and the vessel elements are dead at maturity The secondary walls of tracheids and vessel elements are often interrupted by pits, thinner regions where only primary walls are present…water can migrate laterally between neighboring cells through pits Tracheids are long, thin, and tapered ü Water moves from cell to cell mainly through the pits xylem Vessel elements are wider, shorter, thinner walled and aligned end to end to form long micropipes. ü The end walls of vessel elements have perforation plates that enable water to flow freely through the vessels Secondary walls of tracheids and vessel elements are hardened with lignin, which prevents collapse under tensions of water transport and provides support How water moves upward? Water is pulled upward from roots to shoot in the xylem Transpiration is the process where plants absorb water through the roots and then give off water vapor through stomata in their leaves Transpirational pull is facilitated by cohesion of water molecules to each other (so water column rises unbroken) and adhesion of water molecules to the xylem vascular tissue Phloem There are two types of phloem: sieve-tube elements and companion cells Sieve-tube elements are alive at functional maturity, though they lack organelles such as a nucleus, ribosomes, a distinct vacuole and cytoskeletal elements ü Sieve plates are the porous end walls that allow fluid to flow between cells along the sieve tube Each sieve-tube element has, in close proximity, a companion cell whose nucleus and ribosomes serve both cells (connected to a sieve-tube member by numerous plasmodesmata) Ground Tissues that are neither dermal nor vascular are the ground tissue system Ground tissue includes cells specialized for storage, photosynthesis, and support Three types of ground tissue: 1. Parenchyma 2. Collenchyma 3. Sclerenchyma Ground Parenchyma ü Relatively unspecialized cells that usually lack secondary cell walls (so they have thin flexible primary cell wall) ü Carries out most of the metabolism of the plant, functioning in photosynthesis (so they contain chloroplasts) and food storage (starch) ü Functionally alive ü Most abundant cell type of all ground tissue Collenchyma ü Lack secondary cell walls but have thickened primary cell walls for flexible support (primary cell walls are thicker than in parenchyma) ü Help support young parts of the plant shoot o Young stems and petioles often have strands of collenchyma cells just below the epidermis ü Functionally alive Sclerenchyma Ground ‒ Presence of thick secondary cell walls reinforced with lignin for rigid, sturdy support ‒ Functionally dead at maturity ‒ There are two types: o Sclereids are short and irregular in shape and have thick lignified secondary walls o Fibers are long and slender and arranged in threads Close up of a sclereid Meristematic A plant can grow throughout its life; this is called indeterminate growth Some plant organs cease to grow at a certain size; this is called determinate growth Meristems are growth regions that have perpetual embryonic tissue that allows for indeterminate growth o Apical meristems are located at the tips of roots and shoots and at the axillary buds of shoots § Apical meristems elongate shoots and roots, a process called primary growth Primary growth of roots The root tip is covered by a root cap, which protects the apical meristem as the root pushes through soil Primary growth of shoots Located in buds Terminal (or apical) bud is found at the tip (or apex) of a growing stem Axillary (or lateral) buds are found at points along stem where lateral branches form Secondary Growth Lateral meristems add thickness to plants, a process called secondary growth ü Secondary growth involves formation of rings within plant body as stem increases in thickness Two lateral meristems: vascular cambium and cork cambium – The vascular cambium adds layers of vascular tissue called secondary xylem and secondary phloem – The cork cambium replaces epidermis with periderm Vascular cambium and secondary vascular tissue The vascular cambium is a cylinder of meristematic cells one cell layer thick It develops from undifferentiated parenchyma cells In cross section, the vascular cambium appears as a ring of initials The initials increase the vascular cambium’s circumference and add secondary xylem to the inside and secondary phloem to the outside Cork cambium and the production of periderm The cork cambium gives rise to the secondary plant body’s protective covering, or periderm Periderm consists of cork cambium plus layers of cork cells it produces – Cork replaces the epidermis on woody stems and roots – Cork is waterproof because the cell walls contain suberin (so minimizes water loss in woody plants) Bark consists of all the tissues external to vascular cambium, including secondary phloem and periderm As a tree or woody shrub ages, the older layers of secondary xylem (the heartwood) no longer transport water and minerals The outer layers, known as sapwood, still transport materials through the xylem Older secondary phloem sloughs off and does not accumulate Lenticels in the periderm allow for gas exchange between woody plants and the outside air ü They are visible raised pores that have a blister-like appearance Other differences between monocots and eudicots Eudicot plants can contain flowers in which petals are in multiples of 4 or 5 while monocot plants contain flowers in which petals are in multiples of 3 Eudicot seeds contain two cotyledons while monocot seeds contain one cotyledon – Cotyledon - is a ready-made leaf that is contained in the seed... these leaves are the first to emerge when the seed germinates Eudicot seeds undergo either epigeal or hypogeal germination while monocot seeds only undergo hypogeal germination – Epigeal germination – cotyledons above ground – Hypogeal germination – cotyledons below ground Food storage within a eudicot seed is in cotyledons while food storage within a monocot seed is in the endosperm