Origin• First stem of a plant develops from part of a seed embryo called epicotyl, w/c is a continuation of the hypocotyl
The Plant Body: StemsFUNCTIONS OF STEMS• Produces & support appendages of plant (leaves, flowers, fruits)• transport water and solutes between roots and leaves.• Stems in some plants are photosynthetic.• Produce & store materials necessary for life (e.g., water, starch, sugar).• In some plants, stems have become adapted for specialized functions.
Stems support a display ofleaves.Stems orient the leavestoward the light with minimaloverlap among the leaves.
Apical Dominance Apical dominance refers to the suppression of growth by hormones produced in the apical meristem. Lateral branch growth are inhibited near the shoot apex, but less so farther from the tip. Apical dominance is disrupted in some plants by removing the shoot tip, causing the plant to become bushy.
Typical Stem Cross Section (Dicot Stem) Epidermis Helianthus annuus- sun flower annualCortex A ring of vascular bundles Pith
Epidermis - window, reduce water lossCortex Collenchyma - extensible supportCortex Parenchyma - photosynthesis, etc.Fibers- rigid supportFunctional Phloem - conduct sugars etc. away from leaf to rest of plantVascular Cambium - adds 2° xylem and 2° phloemXylem -conduct water and minerals up from soilPith -water storage, defense?
VIP Stem: Provide both name and function labels: Epidermis: reduce evaporation, gas exchange Cortex: photosynthesis, collenchyma support Vascular Bundles: conduction Pith: water storage? defense? disintegrate? Vascular Bundle:to center outside Phloem Fibers: support Functional Phloem: conduct CH2O away from leaf Vascular Cambium: add 2° Xylem and 2° Phloem Xylem: conduct minerals up from soiloutside to center
Notice how the vascular cambia of adjacent vascular bundles line up side by side. Notice that cambium tissue differentiates between the bundles, connecting the cambia together. Remnants of the procambium: Intrafasicular cambium Interfasicular cambiumVitis vinifera - grape
The vascular cambium makes 2° tissues: Vitis vinifera - grape 2° xyl 2° ph em loe m
Three years of Secondary Growth Tilia - basswood Secondary c ambium Phloem Secondary Xylem
A cork cambium differentiates and produces a periderm. Epidermis cutin suberin Cork Cells Cork Cambium Phelloderm
Over time, the epidermis dies.The cork cells build up to fora thick layer for the bark of atree. We use this to makestoppers for wine bottles andso on.When suberin is fullydeveloped, the cortex cellswill eventually be in thedark. So these chloroplastswill lose their function!
Bark = epidermis + periderm + cortex + phloem + vascular cambiumWood = secondary xylem only!Pith = a small percentage of tree diameter at maturity
The trees pictured below have long lost theirepidermis on the woody portion of the stem Sequoia sempervirens - giant sequoia
The study of the growth rings in wood: Dendrochronology
Each year the cambiumproduces a layer of secondaryxylem and a layer of secondaryphloem.This photo shows secondaryxylem from parts of three yearsin Pinus strobus (white pine). spring of the next year winter of that year fall of that year mid-summer of one year
StemsStems—the axes of plants—consist of nodes (whereleaves and axillary buds are produced) separated byinternodes.
Node - region of the stem where the leaf and bud areborne.Internode - the part of the stem between two adjacentnodes.Herbaceous - not woody; dying down at the end of thegrowing season.Woody - hard in texture, containing secondary xylem, andpersisting more than one growing season.Acaulescent - having an inconspicuous stem.Caulescent - having a distinct stem.
Bulbs• Bulbs - large buds with a small stem at the lower end surrounded by numerous fleshy leaves that store nutrients; adventitious roots at base• Eg. onion, tulip, hyacinth, daffodil and lily
Corms• Corms - resemble bulbs but composed entirely of stem tissue surrounded by a few papery scale like leaves, food storage organs with adventitious roots at the base of corms• Eg. crocus and gladiolus.
Rhizomes• Rhizomes - horizontal stems that grow below the ground with adventitious roots• Eg. irises, ferns, and grasses.
Tubers• Tubers – swollen regions of stems that store food for subsequent growth• The "eyes" of a potato (irish potatoes Solanum tuberosum) are the nodes of a starch-ladened stem
Stolons• Stolons or runners - Bermuda grass (Cynodon dactylon). horizontal stem that grow above the ground with long internodesEg. Bermuda grass (Cynodon dactylon) Spider plant (Chlorophytum) Fern (Nephrolepis)
Rosette• Rosette - stem with short internodes and leaves attached at nodes
Wild Radish – Rosette & Bolt A FLOWERING ANNUAL YEAR ONE YEAR ONE
Common Mullen – Rosette & Bolt A FLOWERING BIENNIAL YEAR ONE YEAR TWO
BudsBuds are short embryonic stems. Inangiosperms they are found at the nodes,in the leaf axil (the angle formed by thestem and the petiole of the leaf).Axillary bud - a bud located in the leafaxil.Terminal bud - a bud at the apex of astem.
PHYSIOLOGY OF STEMS• Conduction of Materials by Xylem 1) Root pressure – powered by transpiration of water from the leaves 2) Transpiration pull and water cohesion – water is pulled up from the roots due to adhesion of water to the xylem walls & tension generated by the water-potential gradient bet. leaves & xylem Other contributing factors: 3) Atmospheric pressure 4) Action of Living cells 5) Imbibition in cell walls of xylem 6) Capillary attraction
PHYSIOLOGY OF STEMS• Conduction of Materials by Phloem - nutrient-rich fluid in the phloem moves from areas of high solute concentration & water pressure to areas of low solute concentration & water pressure• Hypotheses of phloem function are: 1) cytoplasmic streaming 2) movement through interface 3) pressure flow or mass flow
Transpiration-Cohesion Hypothesis for Water Movement