31. Plant
Organs
31.1.
Plant Organs
A. Flowering Plant Structure
1. Structures of flowering plants are well-adapted to varied environments.
2. Flowering plants have three
vegetative organs: root, stem and leaf. (Figs. 36.1 and 36.2)
B. Roots
1. We speak of "a root" but it is more appropriate to call it a root
system.
2. Root system is the
main root plus its lateral (side) branches.
3. It is generally equal in size to
the shoot system, the part above round.
4. Root systems have the following
functions.
a. Roots
anchor a plant in soil and give support.
b. Roots
absorb water and minerals from soil; root hairs are central o this process.
1) Root hair cells are in a zone near root tip.
2) Root hairs are numerous to increase absorptive surface of a root.
3) Transplanting plants without preserving root hairs damages a plant.
4) Water and nutrients absorbed are distributed to rest of the plant.
5) Roots produce hormones that must be distributed to the plant
c.
Perennials "die back" to regrow next
season; roots store food. (e.g., carrots, sweet potatoes).
C. Stems
1. Stem forms main axis of a plant, along with lateral branches.
2. Stem produces leaves and arrays
them to be exposed to as much sun as possible..
3. A node occurs where
a leaf attaches and internode is region
between nodes; nodes and internodes
identify a
stem even if it is underground.
4. Stem has vascular tissue to
transport water and minerals from roots and sugar from leaves.
5. Non-living cells form a
continuous pipeline in vascular tissue.
6. A cylindrical stem expands in
girth and length; trees use woody tissue to strengthen stems.
7. Stems function in storage: cactus
stems store water; tubers are horizontal stems that store nutrients.
D. Leaves
1. A foliage leaf is the usual organ of photosynthesis in
vascular plants. 2. Leaves receive
water from roots by way of the stem.
3. Broad, thin leaves have maximum
surface area to absorb CO2 and collect solar energy.
4. A blade is the wide
portion of a leaf with most photosynthetic tissue.
5. Petiole is a stalk
that attaches a leaf blade to stem.
6. Leaf axil
is upper acute angle between petiole and stem where an axillary
(lateral) bud originates.
7. Some leaves protect buds, attach
to objects, store food, or capture insects.
31.2.
Monocot Versus Dicot Plants
A. Criteria for Monocots and Dicots
1. Flowering plants are divided into monocots and dicots based on these traits. (Fig. 36.3)
|
|
Monocots |
Dicots |
|
a. Number of cotyledons in
seed |
one |
two |
|
b. Distribution of root
xylem and phloem |
root xylem and phloem in a
ring |
root phloem between arms of
xylem |
|
c. Distribution of vascular
bundles |
scattered in stem |
arranged in a distinct ring |
|
d. Pattern of leaf veins |
form a parallel pattern |
form a net pattern |
|
e. Number of flower parts |
in threes and multiples of
threes |
in fours, fives and
multiples of fours or fives |
|
f. Number of apertures in
pollen |
usually one |
usually three |
|
2. Representative members: |
grasses, lilies, orchids |
dandelions to oak trees and
palm trees |
3. Cotyledons are embryonic seed leaves providing nutrition
before mature leaves begin photosynthesis.
4. These traits represent an
evolutionary path dating back to origin of flowering plants.
31.3. Plant
Tissues
A. Meristem Produces Tissue
1. Plants continually grow due to meristem
(embryonic tissue) in stem and root tips.
2. Three types of primary meristem produce three types of specialized tissue.
a. Protoderm is outermost primary meristem giving rise to epidermis.
b. Ground
meristem is inner meristem
producing ground tissue.
c. Procambium produces vascular tissue.
3. Three specialized tissues are
produced.
a. Epidermal
tissue forms outer protective covering.
b. Ground
tissue fills the interior.
c. Vascular
tissue transports water and nutrients and provides support.
B. Epidermal Tissue
1. Epidermis is an outer protective covering tissue of plant
roots, leaves, and stems of nonwoody plants.
2. It contains closely packed
epidermal cells.
3. Waxy cuticle covers
walls of epidermal cells, minimizing water loss and protecting against
bacteria.
4. In roots, certain epidermal cells
are modified into root hairs that increase surface area of the
root for
absorption
of water and minerals and help to anchor plants.
5. Different protective hairs are
produced by epidermal cells of stems and leaves.
6. Epidermal cells are modified as
glands to secrete protective substances.
7. On lower epidermis of leaves,
special guard cells form microscopic pores (stomates)
and regulate gas
exchange
between leaf interior and exterior.
8. In older woody plants, epidermis
of the stem is replaced by cork tissue.
a.
b. Cork
cambium is lateral meristem that produces new
cork cells.
c. As cork
cells mature, they encrust with lipid suberin
that renders them waterproof and inert.
d.
C. Ground Tissue
1. Ground tissue fills the inside of plants with parenchyma, collenchyma and sclerenchyma
cells.
2. Parenchyma are
least specialized of all plant cell types.
a. Cells of
this type contain plastids (e.g., chloroplasts or colorless storage plastids).
b. They are
found in all organs of a plant.
c. They
divide to form more specialized cells (e.g. roots develop from stem cuttings in
water).
3. Collenchyma
resemble parenchyma but has thicker primary cell
walls.
a. Collenchyma cells are uneven in the corners.
b. They
usually occur as bundles of cells just beneath epidermis.
c. They give
flexible support to immature regions of plants (e.g. celery stalk is mostly collenchyma).
4. Sclerenchyma
cells have thick secondary cell walls.
a. They are
impregnated with lignin that renders walls tough and hard.
b. They
provide strong support to mature regions of plants.
c. Most
cells of this type are non-living.
d. Sclerenchyma cells form fibers (used in linen and rope) and
shorter sclereids (found in seed coats
and nut shells).
D. Vascular Tissue
1. Xylem passively conducts water and mineral solutes upward
through a plant from roots to leaves;
a. Xylem
contains tracheids, vessel
elements, and parenchyma cells that store various substances.
b. Tracheids
1) Tracheids are hollow, thin, long non-living cells
with tapered overlapping ends.
2) Water moves across end and sidewalls because of pits or depressions in
secondary cell wall.
c. Vessel Elements
1) Vessel elements are hollow non-living cells lacking tapered ends.
2) They are larger than tracheids.
3) They lack transverse end walls.
4) They form a continuous pipeline for water and mineral transport.
d. Xylem
also contains sclerenchyma cells to add support.
2. Phloem is vascular
tissue that conducts the organic solutes in plants, from the leaves to the
roots; it
contains sieve-tube
cells and companion cells.
a. Sieve-tube
cells
1) Sieve-tube cells contain cytoplasm but no nucleus.
2) They are arranged end to end.
3) They have channels in their end walls (thus, the name
"sieve-tube"), through which plasmodesmata
extend from one cell to another.
b. Companion
Cells
1) Companion cells are closely connected to sieve-tube cells by numerous plasmodesmata.
2) They are smaller and more generalized than sieve-tube cells.
3) They have a nucleus which may control and maintain the function of both
cells.
4) They are also thought to be involved in the transport function of phloem.
3. Vascular tissue extends from root
to leaves as vascular cylinder (roots), vascular bundles (stem) and leaf veins.
31.4.
Organization of Roots
A. Dicot Root Tip
1. Dicot root tip, site of primary growth,
is organized into zones of cells in various stages of differentiation.
2. Cells are continuously added to a
root cap below and zone of elongation above by
contributions from
the zone
of cell division.
3. Root cap is a
protective cover; its cells are replaced constantly because they are soon
ground off.
4. Zone of elongation is
above a zone of cell division where cells become longer and more specialized.
5. Zone of cell division
contains meristematic tissue and adds cells to root
tip and zone of elongation.
6. Zone of maturation
is above zone of elongation; cells are mature and differentiated and it has
root hairs.
B. Tissues of a Dicot Root
1. Epidermis is a single layer of thin-walled, rectangular cells.
a. Epidermis
forms protective outer layer of the root.
b. In region
of maturation, there are many root hairs.
c. Root
hairs project as far as 5-8 mm into the soil.
2. Cortex is a layer
of large, thin-walled, irregularly shaped parenchyma cells.
a. These
cells contain starch granules; function in food storage.
b. Cells are
loosely packed; water and minerals can diffuse through cortex without entering
cells.
3. Endodermis is layer
of rectangular cells; forms boundary between cortex and inner vascular
cylinder.
a. Its cells
fit closely together and are bordered on four sides by the Casparian
strip.
b. It
regulates entrance of minerals into vascular cylinder.
c. Casparian strip is impermeable lignin and suberin layer that excludes water and mineral ions.
d. The only
access to the vascular bundle is through endodermal
cells.
4. Vascular cylinder
is an arrangement of vascular tissues as a cylinder. (Fig. 36.8b)
a. Pericycle is first layer of cells within
vascular cylinder
1) Its cells have retained capacity to divide.
2) It can start development of branch or secondary roots.
b. Vascular
tissue forms main portion of a vascular bundle; it is composed of
1) xylem, whose cells are arranged in a star-shaped pattern; and
2) phloem, whose cells are located in regions between arms of
xylem.
C. Organization of Monocot Roots
1. Monocot roots do not undergo secondary growth.
2. Monocot root has a ring of
vascular tissue; alternating bundles of xylem and phloem surround pith.
3. Monocot roots also have pericycle, endodermis, cortex, and epidermis.
D. Root Diversity
1. Roots have adaptations to help anchor plants, absorb water and minerals, and
store carbohydrates.
2. There are three general root
types:
a. Taproot
is common in dicots; first or primary root
grows straight down and remains dominant root of
a plant; often fleshy and adapted to store food (e.g., carrots, beets). (Fig.
36.11a)
b.
Fibrous root system of monocots is a mass of slender roots and lateral
branches that hold soil.
c. Adventitious
roots develop from underground stems or from base of above-ground
stems.
1) Prop roots main function is to anchor a plant (e.g. corn and
mangrove plants).
2) Pneumatophores of mangrove plants
project above the water from roots to acquire oxygen.
3) Ivy has holdfast roots to anchor aerial shoots.
3. Haustoria
are root-like projections of stems of parasitic plants (e.g., dodders and
broomrapes).
a. They grow
into the host plant.
b. They
contact vascular tissue from which they extract water and nutrients.
4. Mycorrhizae
are fungus roots.
a. In this
mutualism, fungus receives sugars and amino acids from plant.
b. Plant
receives water and minerals from the fungus.
5. Legumes (e.g., peas and beans)
have root nodules containing nitrogen-fixing bacteria.
a. Bacteria
extract nitrogen from air and reduce it to a form that can be used by plant
tissues.
b. Legumes
are often planted to bolster nitrogen supply of soil.
31.5.
Organization of Stems
A. Primary Growth
1. Stem tip is site of primary growth where cell division extends
length of stems or roots.
2. Shoot apical meristem produces new leaves and primary meristems; increases stem length.
3. Shoot apical meristem
is protected within a terminal bud of leaf primordia
(immature leaves).
4. Bud scales are
scale-like coverings protecting terminal buds during winters when bud growth
stops.
5. Three specialized types of
primary meristem develop from shoot apical meristem.
a. Protoderm is outermost primary meristem that gives rise to epidermis.
b. Ground
meristem produces two tissues composed of
parenchyma cells: pith and cortex.
c. Procambium is inner meristem
that produces primary xylem and primary phloem.
6. Differentiation continues; cells
become first tracheids or vessel elements within
vascular bundle.
7. First sieve-tube cells are
short-lived and do not have companion cells.
8. Mature phloem develops later
after all surrounding cells have stopped expanding.
B. Herbaceous Stems
1. Herbaceous stems are mature nonwoody
stems that exhibit only primary growth.
2. Outermost tissue of herbaceous
stems is epidermis covered by waxy cuticle to prevent water loss.
3. Xylem and phloem are in
distinctive vascular bundles.
a. In each bundle, xylem is found to inside of stem; phloem
is found to outside.
b. In dicot herbaceous stem, vascular bundles are arranged in a
ring towards outside of the stem and
separating cortex from central pith.
c. In
monocot stem, vascular bundles are scattered throughout the stem; there is no
well-defined cortex
or pith.
4. Cortex sometimes carries on
photosynthesis; pith may function as a storage site.
C. Woody Stems
1. Woody plants have both primary and secondary tissues.
2. Primary tissues are new and form
each year from primary meristem right behind apical meristem.
3. Secondary tissues develop from
second year onward from growth of lateral meristem.
4. Primary growth increases length
of a plant; secondary growth increases its girth.
5. As secondary growth continues, it
is not possible to distinguish individual vascular bundles.
6. Woody dicot
stem has a different organization with three distinct areas: bark, wood, and
pith.
7. Rays are of living cells that
allow materials to move laterally.
8. Bark of a tree contains cork,
cork cambium, and phloem.
a. Secondary
phloem is produced each year by vascular cambium but does not build up.
b. This
phloem tissue is soft; therefore it is easy to remove the bark of a tree..
9. Cork cambium is a
lateral meristem that produces new cork cells when
needed.
a. Cork
cambium begins to divide, producing cork that disrupts epidermis replacing it
with cork cells.
b.
c.
Consequently, cork forms an impervious barrier, even to gas exchange, except at
lenticels,
10. First flowering plants were
probably woody shrubs; herbaceous plants evolved later.
a. It is
advantageous to be woody when there is adequate rainfall; woody plants can grow
taller and have
adequate tissue to support and service leaves.
b. It takes
energy to support secondary growth and prepare plant for winter in temperate
zones.
c.
Long-lasting plants need more defense mechanisms against attack by herbivores
and parasites.
d. Trees
need years to mature before reproducing; they are more vulnerable to accident
or disease.
11. Annual Rings
a. Vascular
cambium is dormant during winter.
b. Spring
wood is composed of wide xylem vessel elements with thin walls,
necessary to conduct of
sufficient water and nutrients to supply abundant growth that occurs during
spring.
c. Summer
wood forms when moisture is scarce; composed of a lower proportion of
vessels, it
contains thick-walled tracheids and numerous fibers.
d. Annual
ring is one ring of spring wood followed by a ring of summer wood;
equals one year's growth.
e. Sapwood
is outer annual rings where transport occurs.
f. Heartwood
is inner annual rings of older trees.
1) Vessels no longer function in transport; they become plugged with resins and
gums that inhibit
growth of bacteria and fungi.
2) Heartwood may help to support a tree.
D. Stem Diversity
1. Stolons are stems that grow along
ground; new plants grow where nodes contact soil.
2. Succulent stems of cacti are
modified for water storage.
3. Tendrils of grapes and morning
glories are stems adapted for wrapping around support structures.
4. Rhizomes are
underground horizontal stems.
a. Rhizomes
are long and thin in grasses and thick and fleshy in irises.
b. Rhizomes
survive winter and contribute to asexual reproduction because each node bears a
bud.
c. Some
rhizomes have tubers that function in food storage (e.g.,
potatoes).
5. Corms are bulbous
underground stems that lie dormant during winter, like rhizomes.
6. Humans use stems: sugarcane is
primary source of table sugar; cinnamon and quinine are from bark.
31.6.
Organization of Leaves
A. Leaf Structure
1. Leaves are organs of photosynthesis in plants; they are made of a flattened
blade and a petiole.
2. Leaf veins reveal the presence of
vascular tissue within the leaves.
3. The vascular tissues of leaves
transport water and nutrients.
4. Leaf veins have a
net pattern in dicot leaves and a parallel pattern in
monocot leaves.
5. A petiole is a
stalk that attaches a leaf blade to the plant stem.
6. Epidermis is the
layer of cells that covers the top and bottom sides of a leaf.
a. Epidermis
often bears protective hairs or glands; epidermal glands produce irritating
substances.
b. Epidermis
is covered by a waxy cuticle that keeps the leaf from drying out.
c.
Epidermis, particularly lower epidermis, contains stomates
that allow gases to move into and out of leaf.
7. Mesophyll
is the inner body of a leaf and the site of most of photosynthesis.
a. Palisade
mesophyll is layer of mesophyll
containing elongated parenchyma cells with many chloroplasts.
b. Spongy
mesophyll contains loosely packed parenchyma
cells that increase surface area for gas exchange.
B. Leaf Diversity
1. Simple leaves have margins not deeply lobed or divided into
smaller leaflets.
2. Compound leaves are
divided into smaller leaflets, and each leaflet may have its own stalk.
3. Leaves are variously modified.
a. Cactus
spines are modified leaves; succulents have fleshy leaves to hold moisture.
b. Onion
bulbs have leaves surrounding a short stem.
c. Tendrils
of peas and cucumbers are leaves.
d.
Venus's-flytrap has leaves to trap and digest insects.