LEARNING OBJECTIVE 1LEARNING OBJECTIVE 1
• Describe three functions of Describe three functions of stemsstems
Stem Functions Stem Functions
• Support Support • leaves and reproductive structures leaves and reproductive structures
• Conduct Conduct • water, dissolved minerals, carbohydrates water, dissolved minerals, carbohydrates
• Produce new living tissues Produce new living tissues • at apical meristems at apical meristems • at lateral meristems (secondary growth)at lateral meristems (secondary growth)
LEARNING OBJECTIVE 2LEARNING OBJECTIVE 2
• Relate the functions of each tissue in Relate the functions of each tissue in an an herbaceous stemherbaceous stem
Tissues in Herbaceous Stems 1Tissues in Herbaceous Stems 1
• EpidermisEpidermis • protective outer layer protective outer layer • covered by water-conserving covered by water-conserving cuticlecuticle
• Vascular tissuesVascular tissues• XylemXylem conducts water and dissolved minerals conducts water and dissolved minerals • phloemphloem conducts dissolved carbohydrates conducts dissolved carbohydrates
(sucrose)(sucrose)
Tissues in Herbaceous Stems 2Tissues in Herbaceous Stems 2
• Storage tissuesStorage tissues• Cortex Cortex andand pith pith• ground tissue ground tissue
LEARNING OBJECTIVE 3LEARNING OBJECTIVE 3
• Contrast the structures of an Contrast the structures of an herbaceous eudicotherbaceous eudicot stem and a stem and a monocot monocot stemstem
Herbaceous Eudicot StemsHerbaceous Eudicot Stems
• HaveHave vascular bundlesvascular bundles arranged in a arranged in a circle (in cross section)circle (in cross section)
• Have a distinct cortex and pith Have a distinct cortex and pith
Monocot StemsMonocot Stems
• Have scattered Have scattered vascular bundlesvascular bundles
• Have ground tissue instead of distinct Have ground tissue instead of distinct cortex and pithcortex and pith
Epidermis Groundtissue
Vascularbundles
Cornseedling
(a) Cross section of a corn (Zea mays)stem, showing the scattered vascularbundles.
(b) Close-up of a vascular bundle. The air space is where the first xylem elements formed. The entire bundle is enclosed in a bundle sheath of sclerenchyma for additional support.
Bundlesheath
Airspace
Xylem
Phloem
Groundtissue
Fig. 7-4, p. 134
LEARNING OBJECTIVE 4LEARNING OBJECTIVE 4
• Distinguish between the structures of Distinguish between the structures of stems and rootsstems and roots
Differences Between Differences Between Stems and Roots 1Stems and Roots 1
• Unlike roots, stems have Unlike roots, stems have nodesnodes and and internodesinternodes, , leavesleaves and and budsbuds
• Unlike stems, roots have Unlike stems, roots have root capsroot caps and and root hairsroot hairs
KEY TERMSKEY TERMS
• NODE NODE • Area on a stem where one or more leaves is Area on a stem where one or more leaves is
attached; stems have nodes, but roots do notattached; stems have nodes, but roots do not
• INTERNODEINTERNODE • Stem area between two successive nodesStem area between two successive nodes
KEY TERMSKEY TERMS
• BUD BUD • An undeveloped shoot that contains an An undeveloped shoot that contains an
embryonic meristem embryonic meristem • May be terminal (at tip of stem) or axillary (on May be terminal (at tip of stem) or axillary (on
side of stem)side of stem)
Bud scaleTerminalbud
One year’sgrowth
Terminal bud scale scars
Axillary budLeaf scar
Node
Node
Internode
Lenticels
Terminal budscale scars
Bundle scars
Fig. 7-2, p. 131
Differences Between Differences Between Stems and Roots 2Stems and Roots 2
• Internally Internally • herbaceous roots possess an endodermis herbaceous roots possess an endodermis
and pericycle and pericycle • stems lack a pericycle and rarely have an stems lack a pericycle and rarely have an
endodermisendodermis
LEARNING OBJECTIVE 5LEARNING OBJECTIVE 5
• Outline the transition from Outline the transition from primary primary growthgrowth to to secondary growthsecondary growth in a woody in a woody stem stem
• List the two List the two lateral meristemslateral meristems, and , and describe the tissues that arise from describe the tissues that arise from eacheach
Sunflower
Vascularbundles
PithCortex
Epidermis
Epidermis
CortexPhloemfiber cap
PhloemVascularcambiumXylem
Pith ray
Pith
Vesselelement
Vascu
lar bu
nd
le
(a) Cross section of a sunflower (Helianthus annuus)stem, showing the organization of tissues. The vascularbundles are arranged in a circle.
(b) Close-up of a vascular bundle. The xylem is toward the stem’sinterior, and the phloem toward the outside. Each vascular bundleis “capped” by a batch of fibers for additional support. Fig. 7-3, p. 132
KEY TERMSKEY TERMS
• VASCULAR CAMBIUMVASCULAR CAMBIUM • A lateral meristem that produces secondary A lateral meristem that produces secondary
xylem (wood) to the inside and secondary xylem (wood) to the inside and secondary phloem (inner bark) to the outsidephloem (inner bark) to the outside
Secondary GrowthSecondary Growth
• Occurs in woody eudicots and conifers Occurs in woody eudicots and conifers
• Produced byProduced by vascular cambium vascular cambium • between primary xylem and primary phloembetween primary xylem and primary phloem
Vascular Cambium 1Vascular Cambium 1
• Is not initially a solid cylinder of cellsIs not initially a solid cylinder of cells• becomes continuous when production of becomes continuous when production of
secondary tissues beginssecondary tissues begins
Vascular Cambium 2Vascular Cambium 2
• Certain parenchyma cells between Certain parenchyma cells between vascular bundlesvascular bundles• retain ability to divide retain ability to divide • connect to vascular cambium cells in each connect to vascular cambium cells in each
vascular bundlevascular bundle• form a complete ring of vascular cambiumform a complete ring of vascular cambium
Tim
e
Secondary xylem Secondary phloem
Second division of vascularcambium forms a phloem cell.
Division of vascular cambium formstwo cells, one xylem cell and onevascular cambium cell.
Vascular cambium cell whensecondary growth begins.
Vascular cambium cell
1X
1X
1X
1X
1X
1X2X
2X
2X
2X
1P
1P
1P
1P
1P
2P
2P
2P
3X
3X4X
Fig. 7-6, p. 136
KEY TERMSKEY TERMS
• CORK CAMBIUMCORK CAMBIUM • A lateral meristem that produces cork A lateral meristem that produces cork
parenchyma to the inside and cork cells to the parenchyma to the inside and cork cells to the outsideoutside
• Cork cambium and the tissues it produces Cork cambium and the tissues it produces make up the outer bark of a woody plantmake up the outer bark of a woody plant
Cork CambiumCork Cambium
• Arises near the stem’s surface Arises near the stem’s surface
• Is either a continuous cylinder of dividing Is either a continuous cylinder of dividing cells or a series of overlapping arcs of cells or a series of overlapping arcs of meristematic cells that form from meristematic cells that form from parenchyma cells in successively deeper parenchyma cells in successively deeper layers of the cortex and, eventually, layers of the cortex and, eventually, secondary phloem secondary phloem
(a) At the onset of secondary growth, vascular cambium arises in theparenchyma between the vascular bundles (that is, in the pith rays), forminga cylinder of meristematic tissue (blue circle in cross section).
Pith
Vascular cambium
Cortex
EpidermisPrimaryxylem
Primaryphloem
Fig. 7-5a, p. 135
(b) Vascular cambium begins to divide, forming secondary xylem on the insideand secondary phloem on the outside.
Remnant ofprimary xylem
Remnant of pith
Vascularcambium
Periderm(outer bark)
Secondary xylem(wood)
Secondary phloem(inner bark)
Remnantof epidermis
Remnantof cortexRemnant
of primaryphloem
Fig. 7-5b, p. 135
Secondary xylem(wood)
Remnant ofprimary xylem
Remnant ofpith
Vascularcambium
Secondary phloem(inner bark)
Periderm(outer bark; remnants ofprimary phloem, cortex,and epidermis are graduallycrushed or torn apart andslough off)
(c) A young woody stem. Vascular cambium produces more secondary xylemthan secondary phloem.
Fig. 7-5c, p. 135
(a) LM of cross section of basswood (Tilia americana) stem. Note thelocation of the vascular cambium between the secondary xylem (wood) and secondary phloem (inner bark).
Pith Primary xylemAnnual ring ofsecondary xylem
Secondary xylem (wood)
Vascular cambium
Secondary phloem
Periderm and remnants of primaryphloem, cortex, and epidermis
Expanded phloem ray
Xylem ray
Remains ofepidermis
Cork Corkcambium
Expandedphloem ray
CortexSecondary phloemVascular cambium
Secondary xylem (third year)
Xylem rays
Secondary xylem (first year)Primary xylemPith
(b) Sketch of a pie-shaped segment of the cross section. The primary phloem is not labeled because it is crushed beyondrecognition.
Fig. 7-7, p. 137
Cross sectionof twig Vascular
cambiumPith
Primary xylem
Secondary xylem
Vascular cambiumSecondary phloem
Primary phloem
Phloemfiber cap
Cortex
Fig. 7-8, p. 138
Tilia(basswood)
Long, slender core of woodextracted by a boring tool
Bark
Annual ringsPith
Outerbark
Vascularcambium Annual rings Pith
Outermost ring is the year when the tree was cut.
Matching and overlapping older and older wood sections extends dates
back in time
Sample from an old building in the same area as the forest
Sample from a dead treein the same forest
Sample from a living tree
1931 1926 1920 1918
1940 1932 1931 1926
193219401950
p. 143
Cross sectionof 3-year-oldTilia stem Late summerwood
of preceding year
Springwood
Annualring ofxylem
Latesummerwood
Vascular cambium
Secondaryphloem
Fig. 7-12, p. 144
Annualrings
RaysRays Annual
rings
(c) Radial section
Ray(a) Cross sectionAnnualrings
(b) Tangential section
Fig. 7-13, p. 144
KEY TERMSKEY TERMS
• DEFORESTATIONDEFORESTATION • The temporary or permanent clearance of The temporary or permanent clearance of
large expanses of forests for agriculture or large expanses of forests for agriculture or other usesother uses
KEY TERMSKEY TERMS
• VINE VINE • A plant with a long, thin, often climbing stem A plant with a long, thin, often climbing stem
LEARNING OBJECTIVE 6LEARNING OBJECTIVE 6
• Contrast the various stems that are Contrast the various stems that are specialized for specialized for asexual reproductionasexual reproduction
KEY TERMSKEY TERMS
• RHIZOMERHIZOME • A horizontal underground stem that often A horizontal underground stem that often
serves as a storage organ and a means of serves as a storage organ and a means of sexual reproductionsexual reproduction
• Example:Example: iris iris
KEY TERMSKEY TERMS
• TUBER TUBER • The thickened end of a rhizome that is fleshy The thickened end of a rhizome that is fleshy
and enlarged for food storageand enlarged for food storage • Example:Example: white potato white potato
KEY TERMSKEY TERMS
• BULB BULB • A rounded, fleshy underground bud that A rounded, fleshy underground bud that
consists of a short stem with fleshy leavesconsists of a short stem with fleshy leaves• Example:Example: onion onion
KEY TERMSKEY TERMS
• CORM CORM • A short, thickened underground stem A short, thickened underground stem
specialized for food storage and asexual specialized for food storage and asexual reproductionreproduction
• Example:Example: crocus crocus
Adventitiousroots
Old corm(last year’s)
Corm(modifiedstem)
Axillary bud
Leaf scars
Fig. 7-14d, p. 147
KEY TERMSKEY TERMS
• STOLON STOLON • An aerial horizontal stem with long internodes; An aerial horizontal stem with long internodes;
often forms buds that develop into separate often forms buds that develop into separate plantsplants
• Example:Example: strawberry strawberry
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