Download Stems- Part 1

STEMS
OUTLINE
  External
Form of a Woody Twig
  Stem Origin and Development
  Stem Tissue Patterns
  Herbaceous Dicotyledonous Stems
  Woody Dicotyledonous Stems
  Monocotyledonous Stems
  Specialized Stems
  Wood and Its Uses
STEMS
 Anytime
you use something made of wood,
you’re using something made from the stem
of a plant.
 Stems are linear structures with attached
leaves that provide support and transport of
water and nutrients to the plant body.
 Most stems grow upward, helping to raise
plant structures, such as leaves, off the
ground.
 Some plants also use their stems for
photosynthesis or for food and water
storage.
EXTERNAL FORM OF A WOODY STEM
 Woody
twig consists of an axis with
attached leaves.
  Alternately
or Oppositely arranged.
  Leaves
attached at a node.
  Stem region between nodes is an internode.
  Leaf
has a flattened blade and is usually
attached to the twig by a petiole.
EXTERNAL FORM OF A WOODY STEM
 Axil
- Angle between a petiole and the
stem.
  Axillary Bud- located in axil.
 Terminal Bud often found at twig tip.
 Stipules - Paired appendages at the base
of a leaf. Often remain throughout leaf
life span.
 Deciduous trees and shrubs have dormant
axillary buds with leaf scars left after
leaves fall.
 Bundle scars mark food and water
conducting tissue.
ORIGIN AND DEVELOPMENT OF STEMS
 The
apical meristem is located in a bud,
called a terminal bud, at the tip of the
stem.
 New growth in length occurs because of
cell division in the apical meristem.
 Along with the apical meristem, terminal
buds may contain leaf primordia, fully
formed, tiny embryo leaves that are
ready to expand and grow when the bud
opens.
ORIGIN AND DEVELOPMENT OF STEMS
 Buds
are usually protected by hard,
leaf-like structures called bud scales.
 When the terminal bud begins to grow,
the bud scales fall off, leaving a ring of
scars that go 360 degrees all around the
stem.
 For deciduous trees growing in
temperate climates, you can tell how
many years a woody branch has been
growing by counting the number of
rings of bud scale scars.
ORIGIN AND DEVELOPMENT OF STEMS
 Leaves
grow from the stem at nodes.
 On branches from plants that drop their
leaves, called deciduous plants, leaf scars
show where the leaves used to be
attached.
 If you look closely at leaf scars, you can
see little circular marks within them that
show where the vascular tissue from the
stem ran out into the leaf.
 These vascular scars are called bundle
scars because they mark the places where
bundles of vascular tissue were located.
ORIGIN AND DEVELOPMENT OF STEMS
 The
spaces between nodes on the stem are
called internodes.
 Lateral meristems are located in axillary
buds that are tucked in the angles, or axils,
between the leaves and the stem.
 When an axillary bud begins to grow, its
lateral meristems function just like the
apical meristem producing new growth
through cell division.
 Axillary buds may grow into new branches,
or they may produce flowers.
ORIGIN AND DEVELOPMENT OF STEMS
 All
plants grow by getting taller as the
apical meristem produces new cells at the
tip of the stem.
 The stems of some plants that grow for
more than one year will grow thicker over
time.
 So, two types of growth occur in stems.
ORIGIN AND DEVELOPMENT OF STEMS
 Primary
growth increases the length of the
stem.
 Primary growth results from cell division
in apical meristems and builds herbaceous
stems, which are nonwoody stems.
 Secondary growth increases the width of
the stem.
 Secondary growth results from cell division
of the vascular cambium and builds woody
stems, stems that contain secondary xylem
tissue.
PRIMARY GROWTH
 The
apical meristem is dormant before the
beginning of the growing season.
 When a bud begins to expand, apical
meristem cells undergo mitosis and three
primary meristem tissues develop--  Protoderm
  Procambium
  Ground
meristem
PRIMARY GROWTH
  Protoderm
- Gives rise to
epidermis.
  Procambium - Produces primary
xylem and primary phloem cells.
  Ground Meristem - Produces
tissues composed of parenchyma
cells.
 Pith
 Cortex
PRIMARY GROWTH
 Most
flowering plants and conifers
organize their vascular tissue into
vascular bundles, groups of pipe-like
tissues that run longitudinally through
the roots, stems and leaves.
 In primary stems, vascular bundles have
three components:
  A
layer of thick-walled cells, sunch as
sclerenchyma, that surround the bundle,
protecting the bundle and supporting the
stem.
PRIMARY GROWTH
  A
cluster of phloem cells, located
on the side of the bundle closest to
the outside of the stem. These
cells transport food.
  A cluster of xylem cells, located on
the side of the bundles closest to
the inside of the stem. These cells
transport water and minerals.
PRIMARY GROWTH
 Flowering
plants are divided into two
groups, monocots and dicots, based on
several differences, including differences
in stem organization.
PRIMARY GROWTH- Monocot Stem
PRIMARY GROWTH-Monocot Stem
 Most
monocots grow only by primary
growth, remaining herbaceous throughout
their lives.
 The vascular bundles form in a spiral
arrangement around the stem. They can
appear scattered.
PRIMARY GROWTH-MONOCOT STEM
 Moving
from outside to
inside, monocot stems
are made up of  1. epidermis
  2. ground tissue
with vascular
bundles that appear
to be scattered
randomly around the
stem.
PRIMARY GROWTH-Dicot Stem
 Many
wildflowers and crop
vegetables that are dicots grow only
by primary growth and then grow by
secondary growth.
 Dicot stems are surrounded by a
sheath of epidermal tissue. The rest
of the stem, with the exception of the
vascular tissue, is made up of ground
tissue.
PRIMARY GROWTH-Dicot Stems
 In
young dicot stems, ground tissue is
organized into two layers of tissue:
  The
cortex is made of several layers of
parenchyma cells and is located between
the epidermis and the vascular tissue.
  The pith is the group of parenchyma cells
in the center of the stem.
 Dicots
arrange the vascular bundles in a
ring around the stem.
PRIMARY GROWTH-Dicot Stems
 Moving
from the outside to the
inside, primary dicot stems are made
up of…..
  Epidermis
  Ground
tissue (cortex)
  Vascular bundles
  Ground tissue (pith)
SECONDARY GROWTH
 Whether
or not a plant does secondary
growth depends largely upon how long the
plant lives.
  Annual plants don’t do secondary
growth.
  Perennial plants do secondary growth.
 Another
factor that determines growth
pattern is the type of plant. Monocots
don’t do true secondary growth.
Gymnosperms and dicots both do
secondary growth in a similar way.
SECONDARY GROWTH
 Two
rings of meristematic tissue, called
lateral cambia, produce the new cells that
make up secondary growth.
 The vascular cambium makes new
vascular tissue, called secondary xylem
and secondary phloem.
 As the new cells are made, the vascular
cambium pushes the secondary xylem
toward the inside of the stem and
secondary phloem toward the outside of
the stem.
SECONDARY GROWTH
 The cork cambium (phellogen) produces
ground tissue (phelloderm) toward the inside
of the stem and cork cells (phellem) toward
the outside of the stem.
 Cork cells help protect woody stems because
they are impregnated with a waxy substance
called suberin, which makes the waterproof
and resistant to fire damage, infection, and
insect attack.
 These cork cells eventually replace the
epidermal cells that break away as the stem
thickens.
 Lenticels are spongy areas within the cork
that allow gas exchange.
SECONDARY GROWTH
 The
bark of a tree consists of all the cells
from the vascular cambium to the outside
of the stem. This includes the secondary
phloem, parenchyma, and cork cells.
 The outer bark, which consists of just the
cork cambium and the cells it produces
(cork and parenchyma), is called the
periderm.
SECONDARY GROWTH
SECONDARY GROWTH
  The
vascular cambium and cork cambium both
develop from primary tissue in the stem as a
stem enters into secondary growth.
  The vascular cambium forms from cells between
the xylem and phloem within the vascular
bundles and from parenchyma cells in the spaces
between the bundles. These cells develop into
meristematic cells that join up with each other
to form a ring of cells just one cell thick within
the stem.
  The cork cambium usually forms from a ring of
cells within the cortex of the stem. These cells
develop into meristematic cells that form the
ring of cork cambium within the stem.