Botany Chapter 4

Botany Chapter 4Plant StructureRoots, Stems and LeavesObjectives Section 1, Roots1.Describe the main functions of roots2.Compare & contrast tap roots and fibrous roots3.Describe the pattern of tissue development & growth in roots4.Explain how root structure contributes to its function of absorbing water & minerals5.Compare & contrast the organization of tissues in monocot roots with those of dicots and gymnosperms6.Discuss some variations in root structure that correspond to specialized functions7.Discuss the benefits to plants gained by mycorhizal associations and nitrogen fixing bacteria

The Functions of RootsAnchor the plant in the soilAbsorb and transport water and minerals Receive and store organic materials from the stems and leaves

Types of Roots Tap Root vs. Fibrous Roots

Tap root systems have a main root, usually vertical, with lateral roots and smaller branching rootsFibrous roots have no main root. All roots are approximately equal in size Tap Root DevelopmentThe tap root is derived from the embryonic radicleLateral roots grow from the tap root, and branch roots grow from the lateral rootsTap root systems are characteristic of larger, longer living plants

Fibrous Root DevelopmentIn plants with fibrous root systems, the embryonic radicle dies, and roots are derived from the lower stem (adventitious roots)Lateral roots branch off from the adventitious roots and extend horizontally

Which type of root system would have the advantage . . . In deep soil? In shallow soil?For rapid growth?For larger, taller plants?Initially after a light rain?In periods of drought?In porous soil?In storing food, water and minerals?

Root Growth and Development

Root Growth and DevelopmentGrowth initiates near the root tip (apical meristem)Meristematic initials form a small quiescent center just inside the root capDerivative meristem differentiates into procambium, ground meristem, and protoderm

Primary Root Growth Zones

The quiescent center and fresh derivative cells form the Zone of Cell DivisionOlder derivative cells that are growing rapidly form the Zone of ElongationCells that are fully grown and differentiated form the Zone of MaturationRoot hairs are derived from the zone of maturation

Adaptions for AbsorptionWater uptake occurs mostly in the root hairs, which are each only one cell thick. The fine, threadlike nature of the root hairs maximizes the surface area of contact with the soil and its moistureSome of the water that is absorbed will enter the cytoplasm by osmosisSome of the water will remain outside of the cell, associated with the cellulose of the cell wallTransport to the Vascular BundleWater traveling between cells (intercellular), conducted by the cell walls is ApoplasticWater traveling through the cytoplasm and across cell membranes is Symplastic

Endodermis and Casparian StripIn roots, the vascular tissue is at the core of the root, in the SteleThe endodermis forms a boundary between the Cortex and the steleThe intercellular spaces in the endodermis are sealed off by the Casparian Strip. The casparian strip is impermeable to water flow, preventing Apoplastic flow into the steleWhy does it matter?It matters because apoplastic transport is nonspecific. The cell walls and intercellular spaces are not selectively permeable. Only the cell membranes are.By forcing only symplastic transport (through the cells, not around them) into the stele, the plant can control the materials that enter the vascular tissueAnd, the animated version . . . https://www.youtube.com/watch?v=o32jqyIpoHghttps://www.youtube.com/watch?v=TWNtXw-MWtE

Structural Variations in RootsThe arrangement of vascular tissue varies depending on the type of plantIn Monocots, the xylem and phloem alternate with each other in a ring surrounding a core of parenchymaIn Dicots, the xylem forms a lobed core with phloem between the lobes

16Monocot vs. Dicot Roots (cs)

Specialized Functions (see p. 78)Aerial RootsEpiphytes Absorb water from humid airClimbing plants Anchor the plant to a hostAs prop roots Adventitious roots to stabilize and anchor (usually in monocots)Buttress roots Lateral roots for support in thin soil (usually in dicots)Pneumatophores air roots to provide oxygen (if roots are submerged in water)Storage roots like carrots, radishes, beets

Symbiotic Associations A symbiotic association is a close ecological relationship between two different species, where one depends upon the other. If both benefit, it is called Mutualism.Plants live in close association with a variety of microorganisms (fungi and bacteria). Some are parasitic, but some benefit the plant in specific waysMycorhizal AssociationsMyco = fungus, rhizal = rootFungi are heterotrophs, usually decomposers, but mycorhizal fungi obtain food from the roots of a host plantThe host plant loses some food to the fungus, but benefits because the fungus provides water and minerals in greater amounts than the plant could obtain on its own so both benefitNitrogen Fixing BacteriaPlants are photoautotrophs. They produce sugars by photosynthesisProducing amino acids and nucleotides is a more complex issue because it requires nitrogen, which must be obtained from the soil and in a particular formSoil nitrogen originates from the activity of nitrogen fixing bacteriaLegumes (peas, beans, alfalfa) have nodules full of nitrogen fixing bacteria in their roots Symbiotic AssociationsMycorhizal Associationshttps://www.youtube.com/watch?v=JF-i6cJecm0https://www.youtube.com/watch?v=DrsNuwOnoEM

Legumes, with nodules full of nitrogen fixing bacteriahttps://www.youtube.com/watch?v=4NKGS4bj7cchttps://www.youtube.com/watch?v=9j6Jk8YLgfY

Section 1 Review QuestionsHow do taproot and fibrous root systems differ?Describe cell development and maturation through the zones near a root tipWhat are the functions of the root cap, mucigel, and root hairs?What are the roles of the pericycle and endodermis?What are some examples of specialized adaptions in roots?What are mycorhizae, and how are they beneficial?

Objectives Section 2, Stems8.Discuss models of shoot primary growthZone modelCell layer model9.Discuss the arrangement of vascular bundles in stems10.Compare and contrast the structure of stems in monocots, dicots, and ferns11.Describe patterns of leaf arrangement on stemsAlternateOppositeWhorled12.Discuss specialized functions of modified stems (see page 86 in textbook)

Shoot Primary GrowthPrimary growth results from apical meristem activityPrimary growth in stems follows a similar pattern to root growth:The apical meristem initials form a small ball of slowly dividing cellsThe surrounding meristematic derivatives differentiate into the three primary tissues: protoderm, procambium, and ground meristem

Patterns of stem growthThere are two models of stem growth (the Zone model and the Cell Layer model)There are variations between the two, but they share some fundamentalsFor diagrams of the growth models see page 82 in the text

Stem Growth - The Zone Model Apical meristem will give rise to 3 primary meristems: protoderm, ground meristem, and procambiumThe apical meristem is divided into 3 zones:The central mother zone gives rise to the other 2 zones (peripheral and pith zones)The peripheral zone surrounds the central mother zone, divides rapidly, and can derive all 3 primary meristems laterally and distally to the central mother zoneThe pith zone derives ground meristem only, on the proximal side of the central mother zoneStem Growth: Cell Layer ModelThe outer 2 layers of initials in the apical meristem form the Tunica. These cells divide perpendicular to the surface (anticlinal), and give rise to the protodermThe 3rd layer of meristematic initials and underlying derivatives forms the Corpus. These cells divide in both planes (anticlinal and periclinal) and give rise to the procambium and ground meristem

Simplifying the Growth ModelsThe zone model emphasizes the regions of cell division, while the cell layer model emphasizes the cell divisions themselvesBoth models share the same basic patternDerivative cells at the top and sides of the ball of initials grow laterally to form the Leaf Primordia and Vascular tissue Derivatives at the base of the initials grow to form the pith parenchyma

Vascular Bundles

Vascular tissue in the shoot system forms a consistent pattern of bundlesGenerally, xylem cells are located toward the center of the stem, while phloem is more toward the surfaceOrganization of Vascular bundlesAs in the roots, vascular bundles form distinct organizational patterns that vary depending on the type of plantThe trend from root to stem is a periferal migration of vascular bundles (from middle towards surface)

Monocot vs. Dicot Stems

Vascular Variations in StemsSee diagrams in textbook, page 83BryophytesNo vascular tissueSeedless Vascular Plants Vascular tissue in stems is the same as in rootsMonocotsVascular bundles dispersedDicots & GymnospermsVascular bundles in a ring (eustele) outside the pithLeaf Arrangement on Stems

Modified stems

Modified StemsRhizomesUnderground, horizontal stems - StorageCormsResembles a bulb stores food in an enlarged stem surrounded by thin leavesBulbsStores food in fleshy leaves on a short stemTubersUnderground storage stemStolonsAbove ground horizontal stems for asexual reproductionSection Review p. 87How does stem growth differ from root growth?Describe the basic types of steles in stemsWhat is phyllotaxy?What do the differences between palm trees and wheat plants reveal about stem growth?What are some similarities between modified stems and modified roots?Objectives Section 3, Leaves13.Describe the process of leaf formation14.Identify characteristic parts of a leaf and describe the structure and function of eachExternal Blade, Petiole, StipulesInternalEpidermis, Cuticle, Guard Cells, StomataMesophyll: Palisade and SpongyVascular Bundle: Xylem, Phloem, Bundle Sheath15.Explain variations in leaf structureSimple vs. compound leavesShape variationsVenation pattern variations16.Distinguish between deciduous and nondeciduous (evergreen) trees17.Explain the role of abcission zones in seasonal leaf loss (fall)18.Discuss specialized functions of modified leaves (see page in textbook)

Leaf FormationLeaf ButtressBulges that are the first indications of leaf growthLeaf Primordium Elongate from the leaf buttress and form the petiole and bladePetiole and BladePetioleMay range from stem-like to leaf-likeMay be absent (sessile leaves usually monocots)May have 2 stipules at the nodeBladeUsually flattened Growth rates of ridges near the edge of the leaf blade produce variations in leaf marginsLeaf Primordia

Leaf primordia form from the leaf buttressPrimordia are associated with the apical meristemNote also the axillary buds at the next more mature node. The growth rate of these will depend upon the level of auxin hormoneNote apical dominancePetiole, Blade and Stipules

Stipules

Comparison to Sessile Leaf

Blade Margins

Internal Structure of Leaves

Mesophyll and Vascular Bundles

Palisade vs. Spongy Mesophyll

Stomate and Guard Cells

Identify the Labeled Parts

Spongy mesophyllGuard CellsPalisade mesophyllUpper epidermisStomateLower epidermisSimple vs. Compound LeavesCompound leaves have more than one blade sharing the same petioleA node bearing a simple leaf will have an axillary bud at the juncture of petiole to nodeA blade on a compound leaf will not have axillary buds where the blades attach to the petiole

Compound leaves

Pinnate vs. Palmate

Leaf Shape Variations

Venation Variations

Adaptation to Seasonal ChangeDeciduous trees lose their leaves due to seasonal changes (autumn). Evergreens keep their leaves year roundSome evergreens are well adapted to seasonal change, but others simply live in tropical regions where seasonal change is minimalMost deciduous trees are angiosperms while gymnosperms are often evergreenDeciduous Forest in Autumn

Traditional Evergreens

Adaptation to Seasonal Drought

Pine Leaves

Pine leaves are shaped to minimize surface area in proportion to volumeA thick epidermis and heavy waxy cuticle prevent water lossTropical Evergreens

Ginkgo Trees are Deciduous Gymnosperms

Abscission of Deciduous LeavesDeciduous trees drop their leaves as a result of changes at the point of attachment of the petiole to the stemThe abcission layer forms after the plant has reabsorbed water and other materials from the leafThe abcission layer is basically a protective scar

Abcission

Modified LeavesTendrilsCoiling structures used for attachment by climbing plantsSpinesNote: Thorns are modified stems (arise from axillary buds), spines are modified leavesBractsModified leaves superficially similar to petals, found at the base of flowers

Tendrils

Cactus Spines

Bracts