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Tissues and primary Growths of Stems (Stem Growth and differentiation…
Tissues and primary Growths of Stems
Body of a herb is leaves, stem, and roots
Basic Types of Cells and Tissues
Plants used to be dependent on the mud such as algae
stems are transport and supporting structures
All plants have stems but some are temporary or reduced structures
some stems conserve more water
some plants have shoots
other plants have photosynthetic roots
Parenchyma
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only have primary walls that remain thin
parenchyma tissue is a mss of parenchyma cells
most common type of cell and tissue constituting all soft parts of a plant: soft leaves petals fruits and seeds
chlorenchyma cells are involved in photosynthesis located inside parenchyma cells
Glandular cells secrete nectar, fragrances, mucilage, resin, oils
Transfer cells are parenchyma cells that mediate short distance transport of materials by means of large, extensive plasma membranes capable of holding multiple molecular pumps
Collenchyma
have a primary wall that remains thin in some areas but becomes thickened in others
similar to the structure of clay--plasticity
can reform its shaped
present in elongating shoots like grapes
makes tips stronger and more resistant to breaking
needed in inner tissues for support
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needs a constant water supplying vines
require more glucose bc they have thicker walls
Sclerenchyma
has a primary wall and a thick secondary wall
almost always lignified
very elastic cells, can be deformed and then go back to regular shape
develop from parenchyma cells in mature organs after they have stopped growing and have achieved their proper size and shape
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can provide a leafs shape
supports plant by its strength alone, remain upright and do not droop if wilting
keeps protoplast from expanding when absorbing H2O
Two Types: Conducting sclerenchyma and mechanical sclerenchyma
Mechanical sclerenchyma are divided into fibers and sclereids
Fibers
long and flexible
wood of flowering plants
example to humans would be ribs
Sclerids
short and less isodiametric
brittle and inflexible
shells of walnuts or coconuts or pits of peaches/cherries
brain in skull example
External organization of Stems
Stem is an axis and shoots is the stem plus any leaves flowers or buds that may be present
nodes are where leaves are attached
internodes are regions between the nodes
internodes vary from plant to plant
leaf axil is point just above where leaf attaches
axillary bud that has a mini shoot with a dormant apical meristem and several young leaves
vegetative bud or floral bud
covered in bud scales that are cocky and waxy used for protection
at the extreme tip of stems is the terminal bud
Leaves
arrangement of leaves on stems is called phyllotaxy
its important that leaves do not shade each other
decussate phyllotaxy is the most common arrangement of leaves
Stolons are also called runners
their internodes are long and thin
stolons make new plants after finding a suitable microhabitat
Shoots
bulbs are short shoots that have thick fleshy leaves
corms are vertical thick stems have thin papery leaves
Rhizomes
fleshy horizontal stems that allow a plant to spread underground
bamboo, irises, and canna lilies
Tubers
horizontal like rhizomes but grow for only a short period and are mainly used for storing nutrients
bulky underground plant organ
Internal Organization of Stems: Arrangement of primary tissues
Epidermis
outermost surface of a herbaceous stem
single layer of living parenchyma
interchange of material between a plant and its environment occurs here
prevents loss of water
Cutin makes walls impermeable to water
Cuticle
barrier against fungi as well as small insects
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Guard cells+stomatal pore=stoma
become trichomes called hairs
make it hard for animals to eat them
Cortex
interior to epidermis
varies in complexity
most cortex cells fit together compactly
in fleshy stems, cortex parenchyma is aerenchyma an open tissue with larger intercellular air spaces
Vascular Tissue
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Two types xylem and Phloem
Xylem conducts water and minerals
Phloem distributes sugars and minerals
phloem cells are living
Not equal to a circulatory system
water and nutrients enter xylem in roots and transported to leaves
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Xylem
Tracheids
vessel elements
both vessel elements and tracheids are types of sclerenchyma
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tracheary element refers to either cell
must enter cell cycle arrest first
annular thickens on interior face of primary wall
helical thickenings on secondary wall
scalariform thickening provides much more strength
reticular thickening is net shaped
strongest and derived are circular bordered pits
Phloem
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sieve cells, sieve tube members
sieve element refers to either
Parenchyma cells
plasmodesmata occur in group called sieve pores
remain alive during differentiation
large sieve areas stacked end to end are called sieve tubes
shorter than the plant and the small lateral sieve tubes are important for transport
nuclei degenerate but cells remain alive
associated with albuminous cells and controlled by companion cells
Stem Growth and differentiation
stems grow longer by creating new cells at their tips in regions known as shoot apical meristems
region below apical meristem is subapical meristem
protaoxylem is first xylem to appear
metaxylem is the largest treachery element of all
protoxxylem are dead at maturity they stretch bc their primary walks are glued by their middle lamellas to living elongating cells that surround them
exterior cells mature to become protophloem
closest cells to the metaxylem are metapholem
