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Tissues and the Primary Growth of Stems (External Organization of Stems,…
Tissues and the Primary Growth of Stems
Basic Types of Cells and Tissues
Cells grouped into three classes
parenchyma
only primary walls that remain thin
allows for diffusion of gases and materials
Parenchyma tissue
most common tissue composed of most common cell
all soft parts of the plant
soft leaves, petals, fruits, and seeds are entirely parenchyma
active metabolically
alive after maturation
chlorenchyma cells = parenchyma cells
involved in photosynthesis
many chloroplasts
also other pigmented cells = parenchyma
glandular cells
secretion of nectar, fragrances, resins, and oils
not many chloroplasts, but numerous ER and dictyosomes
transfer cells
increase surface area of plasma membrane of cell wall
transport material
some die at maturity
begins cell function
pea pods and stamens
collenchyma
thin walls in some areas, thick in other areas
plasticity, or ability to be deformed by high pressure and retains shape
location
underneath epidermis
bands near vascular bundles
allows resistance to breaking
require large amounts of glucose to make
work with parenchyma to support
work together like air pressure and a tire
parenchyma on the inside
useless unless confined by something
collenchyma on outside
strong and thick, yet no good without air pressure
sclerenchyma
primary wall and a thick secondary that is lignified
walls are elastic
maintains leaf's shape
supports the plant by its strength alone
rigidity makes unusable in growth of shoot tips
develop from parenchyma cells in mature organs
Two types
conducting
Tracheids
vessel elements
transports water and type of vascular cells
mechanical
subdivided
long fibers
long and flexible
found in areas where strength and flexibility are needed
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short sclereids
less isodiametric (cuboidal)
strong walls in all 3 dimensions
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External Organization of Stems
stem = axis
shoot = stem + leaves + flowers + buds
all flowering plants have the same basic stem organization
nodes= leaf attachment
internodes= regions between nodes
elongated helps a plant explore
stolons
advanced exploration
allows shoots to grow in sunnier spots
leaf axil= stem area above the point where a leaf attaches
axillary bud within
miniature shoot
has apical meristem
grows into branch or flowers
bud scales
small, corky, waxy
protect delicate organs inside
terminal bud
leaf scar= where leaves where once attached
phyllotaxy
arrangement of leaves on the stem
important for not shading other leaves
alternate
leaves one per node
opposite
leaves two per node
decussate
leaves located in four rows
whorled
three or more leaves per node
spiral
leaves not aligned with their nearest neighbors
distichous
leaves located in two rows only
nutrient storage
massive shoots and very fleshy
allows starch accumulation
bulbs
short shoots, with thick, fleshy leaves
onions, daffodils, garlic
corms
vertical, thick stems with thin, papery leaves
crocus and gladiolus
Rhizomes
fleshy horizontal stems that allow a plant to spread underground
bamboo
Tubers
horizontal, but grow for only a short time
mainly a means of storage
potatoes
subterranean locations
Internal Organization of Stems: Arrangement of Primary Tissues
Epidermis
outermost surface
single layer of living parenchyma
interchange of material and environment occur here
functions
protection against invasion
fungi
bacteria
small insects
shields from abrasions
reflectivity
keeps plan from overheating
water loss prevention
Cutin
fatty substance on outer walls
impermeable to water
water retention
builds up as pure layer called cuticle
resist digestive enzymes
protects against fungi and bacteria
inhibit CO2 to enter
totally impermeable= plant starvation
Way around this?
guard cells + stomatal pore = stoma
stay closed during night
can however be opened at night for some plants
guard cells swell with water
allow CO2 in
O2 out
Trichomes
hairs
elongated epidermal cells
make it difficult to walk on, land on, or bite into
shade underlying tissues
capture water molecules
main die after maturity and cell wall remains to act as protection
Cortex
Interior to Epidermis
cells fit compactly
fleshy stems have aerenchyma instead of parenchyma
open tissue with large intercellular spaces
mainly in tubers and corms
aquatic plants
large cortical air chambers for buoyancy
Homogeneous and simple
composed of photosynthetic parenchyma and some collenchyma
or can be complex
contains many specialized cells
secrete latex, mucilage, or resin
some contain crystals of calcium, or deposits of silica
Vascular Tissue
two types
Xylem
conducts water and minerals
water enters through stem and conducted towards leaves and stem
two types of cells
types of sclerenchyma
tracheids
obtain water from tracheids below and pass it to those above
pits aligned adjacent to one another
pit membranes
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vessel elements
move water with less friction
individual cells with primary and secondary walls before maturity
perforation
large hole digested through primary wall
aligned adjacent to one another
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vessel
stack of vessel elements
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either referred as tracheary element
starts as parenchyma with thin primary cell wall
cell soon becomes long and narrow
secondary wall deposited
impermeable to water
primary wall is reached by secondary forming annular thickening
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cell dies, leaving a hollow tubular wall
Phloem
distributes sugars and minerals
pick up sugars where they are abundant and store in fruits or organs
two types of conducting cells
sieve cells
keeps sieve tube alive
companion cells
nucleus with ribosomes
many passageways between companion and sieve for loading and unloading of sugar
sieve tube members
sieve tube
sieve plate with pores
used for diffusion of environmental elements and O2
not like animal vascular tissue
vascular bundles
Xylem + Phloem
collateral
strands run parallel
located interior to cortex
arrangement
eudicots
one ring around pith
monocot
distributed out as complex network
scattered, but are in there place for a reason
Stem Growth and Differentiation
Apical meristems
creating new cells at tips
mitosis and cytokinesis
push meristem upwards
lower cells left behind as part of younger stem
primary tissues
tissues produced by apical
primary growth
growth and tissue formation from apical
Subapical meristem
below apical
cells divide and produce for region below
protoxylem
annular or helical secondary walls
metaxylem
largest trecheary elements
visual differentiation occurs
cells stop growing and begin to elongate
vessel elements or tracheids
consist of protoderm + ground meristem + provascular tissue + and more ground meristem
vascular bundle
exterior cells = protophloem
cells are short lived
never become differentiated
sieve areas are small
cells closest to metaxylem = metaphloem
cells differentiate fully
large sieve areas and companion cells
terms
protoderm
epidermal cells in early stages of differentiation
provascular tissues
young cells of xylem and phloem
ground meristem
stages of pith and cortex
