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Tissues and the Primary Growth of Stems (Stem Growth and Differentiation,…
Tissues and the Primary Growth of Stems
External Organization of Stems
Stem is an axis
Flowering plants
Nodes where leaves are attached
Internodes are regions between nodes
Arrangement of leaves=phyllotaxy
Important so leaves don't shade each other
Alternate=1 leave per node
Opposite=2 leaves per node
Decussate=leaves in 4 rows
Whorled=3+ leaves per node
Spiral=no alignment
Leaf axil is just above leaf attachment
Within it is the axillary bud
Vegetative bud=branch
Flowering bud=flowers
Waxy bud scales cover the bud
Extreme tip of stem is a terminal bud
Shoot is the stem plus more
Bulbs are short shoots with thick leaves
Corms are tall, thin leaves
Rhizomes spread underground
Tubers, like rhizomes, but short growth
Basic Concepts of Cells and Tissues
Sclerenchyma
Both a primary & thick secondary wall
Can be deformed, but return back to size
Pits form in secondary wall
Two pits make up a pit-pair
Develop from parenchyma cells
In mature organs after growing stopped
This helps maintain the leafs shape
Do not droop if wilting occurs
Divided into two types
Mechanical
Sclereids
Dead at maturity, isodiametric
Fibers
Some types are dead & some alive
Involved in storage & are long
Conducting
Tracheids
Long & narrow with tapered ends
Dead at maturity
Found in all vascular plants
Vessel elements
Short & wide with perpendicular ends
Dead at maturity
Found in flowering plants
Some nonflowering include ferns, horsetails, & gymnosperms
Collenchyma
Primary walls
Thin in some areas, thick in others
Thicker walls means more required glucose
Present in elongating shoot tips
Example: vining plants like grapes
Exhibits plasticity
Ability to be deformed by pressure or tension
Can retain the new shape
Works together with turgid parenchyma
Counterbalance each other to allow the stem to grow
Parenchyma
Cells have thin primary walls
Typically alive at maturity
Glandular cells are also parenchyma
Secrete nectar, fragrances, etc.,
Contain fewer chloroplasts
Transport larges quantities of sugar & minerals
Chlorenchyma cells are parenchyma cells
Involved in photosynthesis
numerous chloroplasts
Transfer cells
Mediate short distance transport
By a large, extensive plasma membrane
Parenchyma tissue=mass of cells
Phloem
#
Conducts nutrients over long distances
Internal Organization of Stems
Arrangement of Primary Tissues
Vascular Tissue
Two types
Phloem
Distributes sugars & minerals
Xylem
Conducts water & minerals
Xylem
#
Tracheids
Tracheary element=either type
Different types of secondary wall
Annular thickening=small amount as set of rings
Helical thickening=exists as helices to primary wall
Scalariform thickening=underlies primary
Reticular thickening=deposited in the shape of nets
Strongest are those with circular bordered pits
Vessel elements
An entire stack=vessel
Cortex
Interior to the epidermis
Can be simple or very complex
In tubers, corms, & succulents
The parenchyma is aerenchyma
Open tissue with large intercellular air spaces
Some contain calcium oxalate or silica
Phloem
Sieve tube members
Sieve element=refers to either
Plasmodesmata enlarges=sieve pores
Clusters=sieve areas
Large areas aligned=sieve tube
Areas with large pores=sieve plates
Controlled by companion cells
Sieve cells
Are associated with albuminous cells
Epidermis
#
Single layer of parenchyma cells
Shields internal cells
Encrusted with cutin
Build up is cuticle
Makes impermeable to water
Contains pairs of guard cells
Outside the stomatal pore to constitute a stoma
Open to allow CO2 to enter
Trichomes (aka hairs)
Provide protection
Act as small secretory glands
Can secrete poison, excess salt, & more
Vascular Bundles
Xylem and phloem occur together
Run parallel=collateral
Primary xylem
Primary xylem
Arranged in one ring surrounding the pith
Region of parenchyma
Stem Growth and Differentiation
Stems grow longer creating new cell tips
Known as shoot apical meristems
Just below=subapical meristem
Produces primary tissues
#
This growth=primary growth
First xylem to appear=protoxylem
Develop into metaxylem
Young cells together=provascular tissue
Exterior cells mature as protophloem
Cells closest to metaxylem=metaphloem
Protoderm=developing epidermal cells
Ground meristem=young stages of pith & cortex
Concepts
Plant body
3 parts=stems, leaves, & roots
Secondary plant body
Woody body
Secondary tissues
Ex. oaks and maples
Primary plant body
Herbaceous body
Usually lives less than a year
Tissues=primary tissues
Ex. petunias and corn
Composed of these cells
Contain types of conducting cells
Phloem=parenchyma cells
Present in primary plant body
