Transcript Lecture # 16 Date

Plant Structure and Function
Plant Structure and Growth
Angiosperm Structure
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Three basic organs:
Roots (root system)
fibrous: mat of thin roots
taproot: one large, vertical root
Stems (shoot system)
nodes: leave attachment
internodes: stem segments
axillary bud: dormant, vegetative
potential
terminal bud: apex of young shoot
apical dominance: inhibits axillary buds
Leaves (shoot system)
blade
petiole
Many plants have modified roots
(a) Prop roots
(d) Buttress roots
(b) Storage roots
(c) “Strangling” aerial
roots
(e) Pneumatophores
Many Plants Have Modified Stems
(a)
Stolons. Shown here on a
strawberry plant, stolons
are horizontal stems that grow
along the surface. These “runners”
enable a plant to reproduce
asexually, as plantlets form at
nodes along each runner.
Storage leaves
(d)
Stem
Rhizomes. The edible base
of this ginger plant is an example
of a rhizome, a horizontal stem
that grows just below the surface
or emerges and grows along the
surface.
Node
Root
(b)
Bulbs. Bulbs are vertical,
underground shoots consisting
mostly of the enlarged bases
of leaves that store food. You
can see the many layers of
modified leaves attached
to the short stem by slicing an
onion bulb lengthwise.
(c)
Tubers. Tubers, such as these
red potatoes, are enlarged
ends of rhizomes specialized
for storing food. The “eyes”
arranged in a spiral pattern
around a potato are clusters
of axillary buds that mark
the nodes.
Rhizome
Root
(a) Tendrils. The tendrils by
which this pea plant clings to
a support are modified
leaves. After it has “lassoed”
a support, a tendril forms a
coil that brings the plant
closer to the support.
Tendrils are typically
modified leaves,
but some tendrils are
modified stems,
as in grapevines.
(a) Simple leaf. A simple leaf
is a single, undivided blade.
Some simple leaves are
deeply lobed, as in an
oak leaf.
(b) Compound
leaf. In a
compound leaf,
the
blade consists
of
multiple
leaflets.
Notice that a
leaflet
has no axillary
bud
at its base.
(b)
(c)
(c) Doubly
compound
leaf.
In a doubly
compound
leaf, each leaflet
is divided into
smaller
leaflets.
(a)
Petiole
Axillary bud
Leaflet
(c) Storage leaves. Most
succulents, such as this
ice plant, have leaves
modified for storing water.
Petiole
Axillary bud
Leaflet
Petiole
Axillary bud
(b)Spines. The spines of
cacti, such as this prickly
pear, are actually leaves,
and photosynthesis is
carried out mainly by the
fleshy green stems.
(e)Reproductive leaves.
The leaves of some
succulents produce adventitious
plantlets, which fall off the leaf and
take root in the soil.
(d)Bracts. Red parts of the
poinsettia are often
mistaken for petals but
are actually modified
leaves called bracts
that surround a group of
flowers.
Such brightly
colored leaves
attract
pollinators.
Plant Tissues
1. Dermal (epidermis): single layer of
cells for protection
• cuticle
2. Vascular (material transport)
• xylem: water and dissolved
minerals roots to shoots
• tracheids & vessel elements:
xylem elongated cells dead at
maturity
• phloem: food from leaves to roots
and fruits
• sieve-tube members: phloem
tubes alive at maturity capped by
sieve plates; companion cells
(nonconducting) connected by
plasmodesmata
3. Ground (photosynthesis, storage,
support): pith and cortex
Plant Cell Types
1. Parenchyma
primary walls thin and flexible;
no secondary walls; large
central vacuole; most metabolic
functions of plant (chloroplasts)
2. Collenchyma
unevenly thick primary walls
used for plant support (no
secondary walls ; no lignin)
3. Sclerenchyma
support element strengthened
by secondary cell walls with
lignin (may be dead; xylem
cells); fibers and sclereids for
support
Plant Growth
• Life Cycles
• annuals: 1 year (wildflowers;
food crops)
• biennials: 2 years (beets;
carrots)
• perennials: many years (trees;
shrubs)
• Meristems
• apical: tips of roots and buds;
primary growth
• lateral: cylinders of dividing
cells along length of roots and
stems; secondary growth
(wood)
Primary growth
• Roots
• root cap protection of
meristem
• zone of cell division
primary (apical) meristem
• zone of elongation cells
elongate; pushes root tip
• zone of maturation
differentiation of cells
(formation of 3 tissue
systems)
Primary Tissues of Roots
• Stele the vascular bundle where both xylem and phloem develop
• Pith central core of stele in monocot; parenchyma cells
• Cortex region of the root between the stele and epidermis
(innermost layer: endodermis)
• Lateral roots arise from pericycle (outermost layer of stele); just
inside endodermis, cells that may become meristematic
Primary Tissues of Stems
• Vascular bundles (xylem and phloem)
• Surrounded by ground tissue (xylem faces pith and phloem
faces cortex)
• Mostly parenchyma; some collenchyma and sclerenchyma
for support
Primary Tissues of Leaves
• Epidermis/cuticle (protection; desiccation)
• Stomata (tiny pores for gas exchange and transpiration)/guard cells
• Mesophyll: ground tissue between upper and lower epidermis
(parenchyma with chloroplasts); palisade (most photosynthesis)
and spongy (gas circulation)
Secondary Growth
• Two lateral meristems
1. Vascular cambium
produces secondary xylem
(wood) and secondary
phloem (diameter increase;
annual growth rings)
2. Cork cambium
produces thick covering
that replaces the epidermis;
produces cork cells; cork plus cork
cambium make up the periderm;
lenticels (split regions of periderm)
allow for gas exchange; bark~ all
tissues external to vascular cambium
(phloem plus periderm)