ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Review Questions and Answers		AP BIOLOGY: Chapter Thirty-Five Outline INTRODUCTION Plants Continue Growing Throughout Their Lives Achieve great size Attain great age Genetically identical individuals propagated for generations Plants Have a Fundamental Unity of Structure fig 35.1 ORGANIZATION OF THE PLANT BODY Basic Plant Body Plan Root system Penetrates soil Absorbs water and ions Anchors plant Shoot system Stems: serve as framework to position leaves Leaves: primary location for photosynthesis Flowers and fruit: serve reproductive functions Tissue Types in Plants Vascular tissue: conducts materials throughout plant Xylem: water and dissolved minerals Phloem: materials needed for growth, carbohydrates, amino acids, hormones Ground tissue: tissue in which vascular tissue is embedded Dermal tissue: outer protective covering, surrounded by waxy cuticle EMBRYONIC DEVELOPMENT First Stage Is Active Cell Division Zygote divides repeatedly to form embryo Meristems established at root and shoot apices Differentiation in angiosperms begins immediately after fertilization fig 35.2 Zygote divides transversely along long axis Establishes polarity of embryo Lower pole divides to form suspensor Embryo proper divides to form protoderm, procambium and ground meristem Apical meristems detected after six days Shoot meristem grows upward differentiating leaves, lateral branches Root meristem grows downward differentiating root structures Differentiation in gymnosperms fig 35.3 Cell walls do not initially form between daughter cells After eight divisions, embryonic cell contains 256 nuclei Cell walls then form and differentiation begins Cells near micropyle divide slowly to produce large cell suspensor Smaller, rapidly dividing cells at end give rise to apical meristem Plant Development Significantly Different from Animal Development Pattern of development not affected by chemical signals in egg as in animals Plant embryo will develop normally even if removed from ovule Embryo affected by environment as it alters concentration and distribution of hormones The Establishment of Developmental Patterns Axis with one or two cotyledons or embryonic leaves fig 35.4 Monocots have one cotyledon, dicots have two cotyledons Absorbs food from endosperm during germination May remain within seed at maturity Tip of epicotyl Contains shoot apical meristem fig 35.4a Portion of stem axis that extends above cotyledons Plumule = epicotyl + young leaves Tip of hypocotyl Contains root apical meristem Portion of stem axis that extends below cotyledons Radicle or embryonic root At lower end of hypocotyl develops into primary root If no radicle, axis below cotyledons is called hypocotyl root axis, has apical meristem and root cap Special development in grasses fig 35.4b Plumule enclosed in sheath called coleoptile Radicle enclosed in sheath called coleorhiza The Role of Seed Dormancy Embryo stops developing at certain point Generally arrested after differentiation of meristems and cotyledons Integuments develop into relatively impermeable seed coat Adaptive importance of seed Development postponed until conditions favorable for plant growth Reinitiation of development tied to environmental factors Seed affords protection at most vulnerable developmental stage Dispersal of seeds permits migration and dispersal into new habitats Seed coat protects metabolically inactive embryo Germination cannot occur until water and oxygen reach embryo fig 35.5 May involve cracking of the seed Seeds may remain viable for hundreds of years Special adaptations assure dormancy Tough fruits only open in response to fire Germination occurs in fire-cleared area Burned plants release abundant nutrients for germinating seed Inhibitory chemicals leached from seed coat in presence of water Prior passage through animal intestines assures dispersal Seeds may germinate in areas where plants are thought to be extinct DEVELOPMENT OF THE PLANT BODY Germination First step includes absorption of water Metabolism resumed in presence of water Initial metabolism in seed may be anaerobic Cracking of seed coat allows uptake of oxygen Few plants germinate underwater in total absence of oxygen Additional environmental signals may be required for germination Light of proper intensity and wavelength Stratification, a series of cold days, prevents germination in midwinter Significant fraction of seeds may still remain dormant Called seed pool Provides a genetic reservoir The Mobilization of Reserves Reserves may be stored in embryo or in its endosperm Stored in starch grains of amyloplasts Fats and oils provide additional food reserves Cotyledon modified in cereal grains Forms scutellum that provides first food from its stored reserves Scutellum absorbs food from endosperm Epithelial layer secretes hydrolases to mobilize starch Aleurone layer secretes hydrolytic enzymes Emergence of Root and Shoot Is Extremely Variable Root may emerge first and anchor plant in soil fig 35.5 Cotyledon activity May remain underground May emerge above ground and become photosynthetic Establishment of the Meristems Apical meristems develop from clumps of tissue at apex of shoot and root Primary growth Initiated by apical meristems near tips of roots, shoots Results in elongation and produces primary plant body Made up of primary tissue Comprises soft shoots and roots, or entire plant if herbaceous Secondary growth Involves activity of lateral meristems Vascular cambium: gives rise to secondary xylem and phloem Cork cambium: produces outer layers of bark of roots and shoots Results in thickening of plant body Produces secondary plant body Made up of secondary tissue Comparison of primary and secondary growth zones fig 35.1 How Long Do Plants Live? Woody versus herbaceous plants Woody plants possess extensive secondary growth Herbaceous plants lack or have limited secondary growth May send up new stems from woody underground structures May germinate and flower in only one season Annual plants grow, form flowers and fruit in less than one year, then die Examples: corn, wheat, soybeans Grow rapidly under favorable conditions May form wood like sunflowers, but are generally herbaceous Biennial plants complete life cycles in two years Rosette forms during first year Energy stored in rosette and underground organs Stored energy used to produce flowering stems, called bolting Examples: carrots, cabbage, beets Harvest storage structures in first year, not grown for fruit or seeds Cycle may take more than two years, plants flower only once, then die Perennial plants grow from year to year May be herbaceous like woodland and prairie wildflowers May be woody like trees and shrubs Deciduous plants lose leaves once during year, remain bare Evergreen plants drop leaves throughout year, never bare PLANT CELL TYPES Meristems Composed of small, unspecialized cells that divide continually fig 35.6 One cell remains in meristem, other becomes part of plant body Plant body cells divide further and begin to differentiate Apical meristems give rise to three types of embryonic tissues Called primary meristems, develop into primary plant body fig 35.7 Protoderm: differentiates into epidermis Procambium: differentiates into primary vascular strands Ground meristem: differentiates into ground tissue Parenchyma and Collenchyma Parenchyma cells fig 35.9;20 Somewhat spherical, least specialized cell type Form masses in leaves, stems and roots lacking secondary growth Are alive at maturity, have fully functional protoplast and nucleus Are capable of further division Possess only primary cell walls Laid down while cell still growing Secondary wall deposited inside primary wall of expanded cell Collenchyma cells fig 35.8 Form strands or cylinders beneath epidermis, along leaf veins Elongated cells with unevenly thickened primary cell walls Living at maturity Example: strings of celery leaf stalk Sclerenchyma Possess thick, tough secondary walls May lack living protoplasts at maturity Secondary walls often impregnated with lignin Adds rigidity to cells Cells are thus lignified Common in cells with supportive or mechanical function Two types of sclerenchyma Fibers Long slender cells that form strands Example: strands of flax woven to produce linen Sclerids fig 35.9 Varied in shape, frequently branched Example: gritty texture of pears Xylem Consists of dead, hollow, tubular cells Principal water conducting tissue Conducted in an unbroken stream from roots to leaves Contains various dissolved minerals Provides support for plant body Primary xylem derived from procambium Secondary xylem derived from vascular cambium Conducting elements: tracheids and vessel elements fig 35.10 Vessels found almost exclusively an angiosperms Elongated cells with thick, lignified secondary walls, resemble fibers Not living at maturity Water flows through openings, pits, in secondary walls Tracheids have pits in common side walls Vessel elements have side wall pits and perforated end walls Series of vessel elements called a vessel Vessels conduct water more efficiently than tracheids Vessels evolved from tracheids are specialized for conduction Some fibers evolved from tracheids are specialized for support fig 35.10a Xylem also includes fibers and parenchyma cells Phloem fig 35.11 Principle food conducting tissue Conducting cells: sieve cells and sieve-tube members Both types possess clusters of pores called sieve areas Both types of cell are living, but neither has nucleus Sieve-tube members found in angiosperms Pores may be larger, called sieve plates Occur end-to-end, forming a series called sieve tubes Sieve cells occur in seedless vascular plants and gymnosperms Less specialized than sieve-tube members Pores are all same size More primitive cell type Sieve-tube members associated with companion cells Specialized parenchyma cells fig 35.11 Carry out metabolic functions that maintain sieve-tube members Possess components of normal parenchyma cells, including nuclei Plasmodesmata connect their cytoplasm with conducting cells Also includes fibers and parenchyma cells Epidermis Flattened cells covered by cuticle, originate from protoderm Contains specialized cells Guard cells: paired cells flanking stoma Stomal openings allow passage of photosynthetic gases, water vapor Stoma occur in leaf epidermis, occasionally on stems and fruit fig 35.12 More numerous on lower surfaces Trichomes: epidermal outgrowths Occur in stems, leaves and reproductive organs Surface appears woolly or fuzzy Help regulate heat and water balance Glandular trichomes may secrete sticky or toxic substances Root hairs: tubular, single cells found near tips of roots fig 35.5 Provide intimate contact between root and soil particles Responsible for all absorption in herbaceous plants PLANT ORGANS Roots Have simpler pattern of organization and development than stems Primary growth may exhibit a number of patterns fig 35.13 Dicot roots fig 35.4 Central column of primary xylem with radiating arms Region called vascular cylinder or stele Strands of primary phloem alternate between xylem arms Monocot root Ring of vascular tissue surrounding central cylinder of pith Ring composed of alternating strands of xylem and phloem Not in bundles or scattered throughout root Distinct regions and layers of cells surround dicot root vascular tissues Pericycle is first layer, cells produce lateral roots Next and largest region is cortex fig 35.14 Endodermis is innermost layer of cortex Determines which minerals and nutrients enter vascular system Cells surrounded by thickened waxy band called Casparian strip Epidermis is outermost region and completely surrounds cortex Protects root , produces root hairs that take up water Lacks a cuticle Monocot roots also possess endodermis and pericyle Growth of root apical meristem Division pushes one cell inwards and one outwards in direction root is growing Outward cell growth results in root elongation, formation of root cap Root cap covers and protects apical meristem Cells are loose, slough off facilitating passage through soil Abundant root hairs just behind actively growing region Branching in roots Root branching initiated from behind root apex, deep within tissues Branch roots arise from divisions of pericycle Lateral root primordia grow out through cortex fig 35.15 Develop characteristics of main root, including root cap Secondary growth Initiated by appearance of vascular cambium, a lateral meristem Vascular cambium arises from procambial cells between primary xylem and phloem Connected by areas of cell division in pericycle Produce cylinder of vascular cambium surrounding primary xylem Structure of cells of vascular cambium Elongated, flattened cells with large vacuoles Apical meristem cells are nonelongate with small vacuoles Division produces cells that become secondary phloem(outward) or secondary xylem (inward) Root increases in girth Cells also divide laterally, cambium increases in diameter as root grows Fusiform initials produce xylem, phloem and cambial cells Ray initials produce rays Radial strands of parenchyma Allow lateral movement of water through root or stem Accumulated products of secondary division called wood Production of outer coverings of root Epidermis lost in first year, replaced by cork Periderm formed with considerable secondary growth Composed of cork, cork cambium and phelloderm Differentiation of cork cambium occurs in first pericycle Later arises from patches of parenchyma in secondary phloem Cell division produces cork cells toward outside of root Inner layers contain fatty suberin, makes cork waterproof Cork cells dead at maturity Division inward produces phelloderm Bark comprises all tissues outward of vascular cambium Inner layers are primarily secondary phloem Outer layers are periderm Outermost layers are cork Shoots Primary growth Strands of vascular procambium occur within soft, young stems fig 35.16 Occur as cylinder in outer portion of ground meristem in dicots Are scattered throughout ground meristem in monocots Inner portion of ground tissue called pith Outer portion of ground tissue called cortex Outer layer of cells may contain chloroplasts Stem is green and photosynthetically active Strands of procambium differentiate into vascular bundles fig 35.17 Contain primary xylem and phloem In roots primary xylem and phloem are on alternating arms Procambial strands grow upward into developing leaf primordia One or more vascular bundles diverge at each node Leaf primordia are the first rudimentary leaves fig 35.18 Buds develop in axils of leaves fig 35.1 May elongate to form lateral branches May remain small and dormant Bud growth suppressed by hormone produced in terminal bud Secondary growth Initiated by differentiation of vascular procambium in dicots Derived from parenchyma cells within vascular bundles of stem Cylindrical form due to differentiation of cells between bundles fig 35.19 Special form of secondary growth occurs in very few monocots Vascular cambium produces xylem and phloem in same manner as in roots Cork cambium produces cork and phelloderm Cork renewed constantly by cork cambium fig 35.20 Can be harvested from certain trees Gas exchange in periderm occurs through lenticels on outer bark fig 35.21 Destruction of vascular cambium Girdling: interrupts transport of materials, ultimately kills plant Caused by activities of beavers, beetles and human Wood Composed of accumulated secondary xylem Common wood obtained from stems not roots Heartwood Located near central region of trunk Denser wood, darker in color Sapwood Located nearer the vascular cambium Actively involved in transport Proportion of heartwood to sapwood varies widely Forms concentric annual rings Active division with larger cells at beginning of growing season At end of growing season cells are smaller Division less active during other seasons Discontinuity between cell sizes produces ringed appearance Can estimate climatic conditions from annual rings Rings thicker in years with plentiful water Rings thinner in drought years Can accurately date pieces of wood Derivation of commercially used wood Hardwood produced by dicots Softwood produced by conifers Species of wood identified by microscopic characteristics fig 35.22 Modified stems fig 35.23 Tendrils of grape, Virginia creeper, ivy (peas tendrils are modified leaves) Rhizomes are underground stems, important in vegetative reproduction Stolons, or runners, are above ground horizontal stems Thorns are modified branches in axils of leaves Prickles are sharp outgrowths from epidermis of leaves and stems Tubers are underground storage organs like common potato Potato eyes are buds arising in the axil of a leaf scale Each eye capable of becoming an individual plant Corms are thick, fleshy upright underground stems modified for storage Bulbs are short underground stems bearing thick, fleshy scale leaves Adventitious roots may arise from stems Adventitious shoots may arise from root tissue May occur at great distances from parent plant Characteristic of many plants that produce clumps: quaking aspen New individuals are genetically identical, clones of parent plant Most underground spreading results from rhizomes Leaves General features fig 35.18 Most important light-capturing, photosynthetic organs Exception: stems in cacti Features differ greatly in physical appearance Grow via cell division and enlargement within blade Mesophyll established early in development Cell division and enlargement ceases when leaf is fully expanded External leaf anatomy Blade: flattened portion fig 35.24 Petiole: slender stalk Stipules: paired leaf-like organs near base of petiole Veins: xylem and phloem strands run throughout leaf fig 35.25 Parallel in monocots Netted or reticulate in dicots Simple leaves are undivided, may be deeply lobed Compound leaves consist of distinctly separate leaflets Pattern of placement on stem fig 35.26 Alternate: spirally arranged on stem Opposite: occur in pairs Whorled: more than two leaves attached at one level on stem Leaves attached to stem at nodes Regions between nodes are internodes Structure and organization Mesophyll: masses of parenchyma through which veins run fig 35.27 Palisade parenchyma: columnar parenchyma on one or both sides Spongy parenchyma: parenchyma cells within leaf interior Intercellular spaces are connected to stomata Mesophyll cells packed with chloroplasts Primary site of photosynthesis Xylem brings water and minerals from root to leaf Water passes into mesophyll cells Some water moves immediately into phloem after sugar secretion Some water exists in intercellular spaces and diffuses out of leaf PLANT GROWTH AND DEVELOPMENT Grow Continuously Repeatedly Produce Similar Structure fig 35.8 Apical Meristems Continually Produce Primary Tissues