Download Root system

 This chapter focuses on the plant
body of Angiosperms
• Plants must harvest energy from sunlight and
collect sparse water and mineral nutrients
from the soil.
• Stems, leaves, and roots enable plants
anchored in one spot to capture scarce
resources effectively.
• They can grow throughout their lifetimes, and
can redirect growth to respond to
environmental cues.
Angiosperms first appeared about 140 Mya, and
radiated explosively to became the dominant
plant life.
Angiosperms have three types of vegetative
organs organized into root systems and shoot
systems:
• Roots,
• stems,
• leaves
• Root system: Anchors plant, absorbs water
and minerals, stores products of
photosynthesis
• Extreme branching of roots provides large surface
area for absorption of water and mineral nutrients
• Shoot system: Stems, leaves, flowers.
• Leaves are the main organs of photosynthesis.
• Stems hold and display leaves in the sun; connection
between roots and leaves.
Most (not all) angiosperms are in two major clades.
• Monocots
• narrow-leaved plants such as grasses, lilies, orchids, and
palms.
• Eudicots
• broad-leaved plants such as soybeans, roses, sunflowers,
and maples.
Water and minerals enter through the root
system in most plants.
The root system originates in an embryonic root
called the radicle.
Types of roots:
• Taproot
•
Single, large, deep-growing root and small side roots. The
root may also function as food storage.
• Fibrous roots
• Monocots have a fibrous root system: Many thin roots of
equal diameter originate from the stem at ground level or
below. Have large surface area; cling to soil well
• Prop roots
• Help support stem of some monocots
 Stems elevate and support flowers and leaves
• Leaves are attached to stem at node
• have buds – undeveloped shoots
• Axillary buds
• At angle where leaf meets stem, can
form new branches
• Terminal buds
• At tips, upward growth
• Flowers can also develop there
Stems can be modified:
• Potato tuber is an underground stem. The
“eyes” are axillary buds.
• Many desert plants have enlarged stems that
store water.
• Runners are horizontal stems, roots grow at
intervals and independent plants can arise
from them.
 Most photosynthesis occurs in the leaves.
 The blade is a thin, flat structure; attached
to stem by the petiole. Angle may be
perpendicular to sun’s rays to provide
maximum area for light gathering.
 Some leaves change position during the day to
track the sun.
Leaves can be highly modified.
 Some function as food storage, (e.g., onion
bulbs).
 Leaves of succulents store water. Cacti spines
are modified leaves.
 Climbing plants have modified leaves called
tendrils that wrap around other structures.
Plant cells have:
 Chloroplasts or other plastids
 A central vacuole
 The single central vacuole may account for 90
percent of the cell volume.
 It contains a high concentration of solutes,
including enzymes, amino acids, and sugars.
 As the vacuole expands, it exerts turgor pressure
on the cell wall.
 Turgor pressure keeps plants upright, and is
essential for plant growth.
 Rigid cell walls with cellulose
 Plant cells can communicate via
plasmodesmata—cytoplasm-filled canals that
are traversed by a strand of ER; proteins and
RNA can to move from cell to cell.
 Tobacco mosaic virus encodes a movement
protein that complexes with its RNA. The
complex can move easily via the
plasmodesmata.
 Apical Meristems
• Clusters of undifferentiated cells form at
the tips of the embryonic root and shoot.
• orchestrate development and allow the plant
to form organs throughout its lifetime.
• Contribute to primary growth
 Lateral Meristems
• Only plants with secondary growth have
these
Tissue: Organized group of cells that work
together as a structural and functional unit.
Tissues are grouped into tissue systems.
• Dermal
• Vascular
• Ground
 Dermal tissue system: Forms the epidermis,
usually a single layer of cells.
 Stems and roots of woody plants have a
dermal tissue called periderm.
Epidermal cells can differentiate:
 Stomatal guard cells form stomata (pores) for
gas exchange
 Trichomes, or leaf hairs, provide protection
against insects and damaging solar radiation
 Root hairs greatly increase root surface area,
for uptake of water and mineral nutrients
 The ground tissue system makes up most of
the plant body.
 Functions in storage, support, and
photosynthesis.
 Has three cell types: Collenchyma,
parenchyma, and schlerenchyma.
 Parenchyma cells: Thin walls (only primary),
large central vacuoles.
 They are sites of photosynthesis (in leaves)
and storage, e.g., protein in fruits and starch
in roots.
 Many retain the capacity to divide and can
give rise to new cells, as when a wound results
in cell proliferation.
 Collenchyma cells: Primary cell walls thickened
by pectins, usually elongate shape.
 Provide support in leaf petioles, nonwoody
stems, and growing organs.
 Tissue with collenchyma cells is flexible, it can
bend without snapping.
 Celery “strings” are collenchyma cells.
 Sclerenchyma cells: Thickened secondary
walls. Many undergo apoptosis after secondary
wall is laid down.
 Fibers: Elongated cells provide rigid support;
often in bundles.
 Sclereids may be densely packed as in nut
shells, or in clumps as in stone cells in pears.
 The vascular tissue system: The transport
system.
 Xylem distributes water and minerals taken up
by roots to all parts of the plant. Xylem can
also function in storage and support.
 Cells called tracheary elements that die before
assuming function
 Tracheids line up like hollow straws one on top of
another
 Phloem transports carbohydrates from site of
production (sources) to sites of utilization or
storage (sinks).
 Made up of transport cells called seive tube
elements that are living
 Plants grow in two directions: Toward sunlight,
and toward water and dissolved minerals in
the soil.
 In most animals, growth is determinate—
growth of the individual and all its parts stops
in adult stage.
 Shoots and roots have indeterminate growth—
continuous throughout life.
 Primary growth – Apical Meristems
 Lengthening of shoots and roots, and branching.
 It results in the primary plant body: All non-woody
parts of the plant. Growth lengthens the plant
body.
 Herbaceous plants and monocots consist entirely of
primary plant body.
 Secondary growth – Lateral Meristems
(vascular cambium and cork cambium)
 Increases diameter.
 Trees and shrubs have a secondary plant body
consisting of wood and bark.
 Tissues are laid down as stems and roots thicken.
Grows throughout life of plant.
The primary meristems: protoderm, ground
meristem, procambium.
 Root apical meristem:
 Some daughter cells become the root cap—
protects root tip as it grows through the soil.
They secrete a muco-polysaccharide (slime) as
a lubricant.
 The root cap detects gravity and controls
downward growth of roots.
 3 zones
 Zone of cell division
 Zone of elongation
 Zone of maturation
Figure 34.12 Products of the Root’s Primary Meristems
Figure 34.13 Root Anatomy (A)
Figure 34.13 Root Anatomy (B)
 Stem tissues:
 Shoots are composed of repeating modules
called phytomers; shoots grow by adding new
phytomers.
 New phytomers originate from cells in shoot
apical meristems at stem tips and axillary
buds.
 In young stems, vascular tissue is arranged in
vascular bundles of both xylem and phloem.
 Eudicots: Vascular bundles form a cylinder.
 Monocots: Bundles are scattered.
• Leaves are produced from apical meristems called
vegetative meristems.
• Growth of a leaf is determinate.
• Leaf anatomy is adapted to carry out
photosynthesis, and the exchange of O2 and CO2
with the environment, while limiting water losses.
Two zones of photosynthetic parenchyma cells make
up the mesophyll (part of ground tissue):
 Palisade mesophyll
 Spongy mesophyll
Also includes air space for diffusion of gases for
photosynthesis
Pores called stomata allow gas exchange. They are
opened and closed by guard cells.
• Secondary growth (wood and bark) arises
from two lateral meristems in eudicots.
• Vascular cambium: Elongated cells that divide
often. Supplies cells of the secondary xylem and
secondary phloem.
• Cork cambium produces waxy-walled protective
cells. Some become part of the bark.
Woody twigs illustrate both primary and
secondary growth.
Apical meristems are enclosed in buds
protected by bud scales.
Vascular cambium is initially a single layer of
cells between primary xylem and phloem.
Division of these cells produces secondary
phloem cells toward the outside, and
secondary xylem cells toward the inside.
• As secondary growth continues, the epidermis and outer
cortex are stretched and flake away.
• Cells near the surface of the secondary phloem begin to
divide, forming a cork cambium.
• Cork is composed of thick-walled cells, waterproofed
with suberin. Cork becomes the outermost tissue of the
stem or root.
The cork, cork cambium, and phelloderm form a tissue called
periderm.
• The periderm and secondary phloem— all the tissues external
to the vascular cambium—constitute the bark.
•
Tree trunks from temperate regions show
annual rings that result from seasonal
conditions.
 Spring—water is plentiful, tracheids or vessel
elements produced have large diameter
 Summer—less water, smaller diameter cells
with thicker walls are produced
• Some monocots have thickened stems, such as
palms, but they don’t have vascular or cork
cambiums.
• Not true wood
•
• Palms have a wide apical meristem that
produces a wide stem. Dead leaf bases also
contribute to the stem diameter.
Members of the same plant species can be
remarkably diverse in form.
Seed banks maintain collections from both
cultivated crops and their wild relatives.
Crops and their wild relatives are members of
the same species and can be crossbred. The
progeny will carry new combinations of the
parents’ traits.
A single species, Brassica oleracea (wild
mustard), is the ancestor of many
morphologically diverse crops: Kale, broccoli,
Brussels sprouts, cabbage.
Humans selected seed from morphological
variants in the wild population with the trait
they found desirable.
• The genomes of plants are still
priceless resources today.
• Genetic variation in crop plants and
their wild relatives can be used to
improve our crop plants or adapt
them to changing conditions.
• This is especially important as
human activities change the planet
and lead to extinction of plant
species.
• The Doomsday Vault on the Arctic
island of Spitsbergen and other
seed banks are an insurance policy
against the loss of our most valuable
resource, the genetic diversity
underlying plant form and growth.