Plant Structure, Growth, and
Development
Chapter 35 & 36
The Cells and Tissues of the Plant Body
Cells of angiosperm embryos differentiate early in
development into 3 distinct tissues:
• A. Dermal Tissue: forms the outside covering of
plants
–
–
–
–
–
Epidermis
Cuticle
Cork
Bark
Stomata
• B. Ground tissue: for storage, metabolism and
support. Mostly parenchyma, with specialized
support cells of collenchyma and sclerenchyma
• C. Vascular tissue: phloem and xylem
consists of special conducting cells, along with
support fibers & parenchyma
The Three Tissue Systems: Dermal,
Vascular, and Ground
Dermal
tissue
Ground
tissue
Figure 35.8
Vascular
tissue
“Ground” tissue:
Includes various cells specialized for functions such as
storage, photosynthesis, and support
•parenchyma: cells which occur in all 3 tissue
systems, usually photosynthesis, elongated, loosely
packed, thin, flexible cell walls
•collenchyma: primary wall (in cells) thickened at
corners, irregular shapes, provide support
•sclerenchyma: 2 types, support and strengthen the
plant, thick, even cell walls, dead cells provide
framework for additional cells
1. fibers- elongated, elastic strands or bundles
associated with the vascular tissue
2. sclereids- form hard outer covering of seeds,
nuts, and fruit stones
Parenchyma, collenchyma, and
sclerenchyma cells
PARENCHYMA CELLS
COLLENCHYMA CELLS
80 m
Cortical parenchyma cells
SCLERENCHYMA CELLS
5 m
Sclereid cells
in pear
25 m
Cell wall
Parenchyma cells
60 m
Collenchyma cells
Fiber cells
Figure 35.9
Vascular Tissue
• Xylem
– Conveys water and dissolved minerals
upward from roots into the shoots
• Phloem
– Transports organic nutrients from where
they are made to where they are needed
Water-conducting cells of the
xylem and sugar-conducting cells
of the phloem
WATER-CONDUCTING CELLS OF THE XYLEM
Vessel Tracheids
SUGAR-CONDUCTING CELLS OF THE PHLOEM
Sieve-tube members:
longitudinal view
100 m
Pits
Companion cell
Sieve-tube
member
Sieve
plate
Tracheids and vessels
Vessel
element
Vessel elements with
partially perforated
end walls
Nucleus
30 m
15 m
Tracheids
Cytoplasm
Figure. 35.9
Companion
cell
Vascular tissue
Transports nutrients throughout a
plant; such transport may occur over
long distances
Figure 36.1
• A variety of physical processes
– Are involved in the different types of
transport
4 Through stomata, leaves
take in CO2 and expel O2.
The CO2 provides carbon for
photosynthesis. Some O2
produced by photosynthesis
is used in cellular respiration.
CO2
O2
5 Sugars are produced by
photosynthesis in the leaves.
Light
H2O
Sugar
3 Transpiration, the loss of water
from leaves (mostly through
stomata), creates a force within
leaves that pulls xylem sap upward.
6 Sugars are transported as
phloem sap to roots and other
parts of the plant.
2 Water and minerals are
transported upward from
roots to shoots as xylem sap.
1 Roots absorb water
and dissolved minerals
from the soil.
Figure 36.2
O2
H2O
Minerals
CO2
7 Roots exchange gases
with the air spaces of soil,
taking in O2 and discharging
CO2. In cellular respiration,
O2 supports the breakdown
of sugars.
Transpiration is the evaporation of
water from plant leaves
• Plants lose a large amount of water by transpiration
• If the lost water is not replaced by absorption through the
roots
– The plant will lose water and wilt
• Turgor loss in
plants causes
wilting
– Which can
be reversed
when the
plant is
watered
Figure 36.7
XYLEM: Several factors are at work in
the movement of water and minerals up
a plant stem
• To survive
– Plants must balance water uptake and loss
• Water is pulled upward by negative pressure in the
xylem, caused by losses by transpiration
• Cohesion
• Adhesion
• Osmosis
– Determines the net uptake or water loss by a
cell
– Is affected by solute concentration and
pressure
• Water potential
– Is a measurement that combines the effects of
solute concentration and pressure
PHLOEM
• Organic nutrients are translocated through
the phloem
• Translocation
– Is the transport of organic nutrients in the plant
• Phloem sap
– Is an aqueous solution that is mostly sucrose
– Travels from a sugar source to a sugar sink
• A sugar source
– Is a plant organ that is a net producer of sugar,
such as mature leaves
• A sugar sink
– Is an organ that is a net consumer or storer of
sugar, such as a tuber or bulb
Phloem
• The pressure flow hypothesis explains why
phloem sap always flows from source to sink
• Experiments have built a strong case for
pressure flow as the mechanism of
translocation in angiosperms
EXPERIMENT To test the pressure flow hypothesis,researchers used aphids that feed on phloem sap. An aphid probes with a hypodermiclike mouthpart called a stylet that penetrates a sieve-tube member. As sieve-tube pressure force-feeds aphids, they can be severed from their
stylets, which serve as taps exuding sap for hours. Researchers measured the flow and sugar concentration of sap from stylets at different
points between a source and sink.
25 m
Sievetube
member
Sap
droplet
Aphid feeding
RESULTS
Figure 36.19
Stylet
SieveTube
member
Sap droplet
Stylet in sieve-tube Severed stylet
member
exuding sap
The closer the stylet was to a sugar source, the faster the sap flowed and the higher was its sugar concentration.
CONCLUSION The results of such experiments support the pressure flow hypothesis.
The Plant Body
• Three basic organs
evolved: roots,
stems, and leaves
• They are organized
into a root
system and a shoot
system
Reproductive shoot (flower)
Terminal bud
Node
Internode
Terminal
bud
Shoot
system
Vegetative
shoot
Leaf
Blade
Petiole
Axillary
bud
Stem
Taproot
Lateral roots
Figure 35.2
Root
system
Growth in Meristems
• When plants grow, they add new cells
(cells divide by mitosis) at the tips/ends
of branches and roots
• Apical meristems
– Are located at the tips of roots and in the
buds of shoots
– Elongate shoots and roots through
primary growth
• Lateral meristems
– Add thickness to woody plants through
secondary growth
The Root
–
–
–
–
Is an organ that anchors the vascular plant
Anchors the plant
Absorbs minerals and water
Often stores organic nutrients
In most plants:
The absorption of water
and minerals occurs near
the root tips, where vast
numbers of tiny root
hairs increase the
surface area of the root
Figure 35.3
• Many plants have modified roots
(a) Prop roots
Figure 35.4a–e
(d) Buttress roots
(b) Storage roots
(c) “Strangling” aerial
roots
(e) Pneumatophores
Primary Growth of Roots
The root tip is covered by a root cap, which protects
the delicate apical meristem as the root pushes
through soil during primary growth
Cortex
Vascular cylinder
Epidermis
Key
Root hair
Dermal
Ground
Zone of
maturation
Vascular
Zone of
elongation
Apical
meristem
Root cap
Figure 35.12
100 m
Zone of cell
division
Taproot and Fibrous Root Systems
dicot
monocot
Stems
A stem is an organ consisting of
An alternating system of nodes,
the points at which leaves are
attached
Internodes, the stem segments
between nodes
1) hold leaves up
and aloft for
maximum sun
exposure
STEMS
Terminal bud
Bud scale
Axillary buds
Leaf scar
2) transport
nutrients/water
up/down (connects
leaves to roots)
3) some stems
store food Figure 35.11
Node
This year’s growth
(one year old)
Stem
Internode
One-year-old side
branch formed
from axillary bud
near shoot apex
Leaf scar
Last year’s growth
(two years old)
Growth of two
years ago (three
years old)
Scars left by terminal
bud scales of previous
winters
Leaf scar
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
Stem
(d) 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
Figure 35.5a–d
(b) Bulbs. Bulbs are vertical,
underground shoots consisting (c)
Tubers. Tubers, such as these
mostly of the enlarged bases
red potatoes, are enlarged
of leaves that store food. You
ends of rhizomes specialized
can see the many layers of
for storing food. The “eyes”
modified leaves attached
arranged in a spiral pattern
to the short stem by slicing an
around a potato are clusters
onion bulb lengthwise.
of axillary buds that mark
the nodes.
Rhizome
Root
Tissue Organization of Stems
• In gymnosperms and most dicots
– The vascular tissue consists of vascular bundles
arranged in a ring
Phloem
Xylem
Sclerenchyma
(fiber cells)
Ground tissue
connecting
pith to cortex
Pith
Key
Cortex
Epidermis
Vascular
bundle
Ground
1 mm
Figure 35.16a
Dermal
Vascular
(a) A eudicot stem. A eudicot stem (sunflower), with
vascular bundles forming a ring. Ground tissue toward
the inside is called pith, and ground tissue toward the
outside is called cortex. (LM of transverse section)
In most monocot stems
The vascular bundles are scattered throughout the
ground tissue, rather than forming a ring
Ground
tissue
Epidermis
Vascular
bundles
1 mm
Figure 35.16b
(b) A monocot stem. A monocot stem (maize) with vascular
bundles scattered throughout the ground tissue. In such an
arrangement, ground tissue is not partitioned into pith and
cortex. (LM of transverse section)
Secondary growth adds girth to
stems and roots in woody plants
Secondary phloem
Vascular cambium
Cork
cambium
Cork
Secondary Late wood
Early wood
xylem
Periderm
(b) Transverse section
of a three-yearold stem (LM)
Xylem ray
Bark
0.5 mm
Figure 35.18b
0.5 mm
As a tree or woody shrub ages
The older layers of secondary xylem, the
heartwood, no longer transport water and
minerals
The outer layers, known as sapwood
Still transport materials through the xylem
Growth ring
Vascular
ray
Heartwood
Secondary
xylem
Sapwood
Vascular cambium
Secondary phloem
Bark
Layers of periderm
Leaves
The main photosynthetic organs
of most vascular plants
• Leaves generally consist of
– A flattened blade and a stalk
– The petiole, which joins the leaf to a
node of the stem
In classifying angiosperms
– Taxonomists may use leaf morphology as
(a) Simple leaf. A simple leaf
a criterion
is a single, undivided blade.
Some simple leaves are
deeply lobed, as in an
oak leaf.
Petiole
(b) Compound leaf. In a
compound leaf, the
blade consists of
multiple leaflets.
Notice that a leaflet
has no axillary bud
at its base.
(c) Doubly compound leaf.
In a doubly compound
leaf, each leaflet is
divided into smaller
leaflets.
Figure 35.6a–c
Axillary bud
Leaflet
Petiole
Axillary bud
Leaflet
Petiole
Axillary bud
Monocots and dicots
Differ in the arrangement of veins,
the vascular tissue of leaves
Most monocots
Have parallel
veins
Most dicots
Have branching vein
“network”
Some plant species
Have evolved
modified
leaves that
serve
various
functions
Figure 35.6a–e
(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.
(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.
(c) Storage leaves. Most succulents,
such as this ice plant, have leaves
modified for storing water.
(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.
(e) Reproductive leaves. The leaves
of some succulents, such as Kalanchoe
daigremontiana, produce adventitious
plantlets, which fall off the leaf and
take root in the soil.
Leaf anatomy
Guard
cells
Key
to labels
Dermal
Ground
Vascular
Cuticle
Stomatal pore
Epidermal
cell
Sclerenchyma
fibers
50 µm
(b) Surface view of a spiderwort
(Tradescantia) leaf (LM)
Stoma
Upper
epidermis
Palisade
mesophyll
Bundlesheath
cell
Spongy
mesophyll
Lower
epidermis
Cuticle
Guard
cells
Xylem
Guard
Phloem
(a)
Cutaway drawing of leaf tissuescells
Vein
Figure 35.17a–c
Vein Air spaces Guard cells
100 µm
(c) Transverse section of a lilac
(Syringa) leaf (LM)
Leaf anatomy
• The outer surface of the leaf has a thin waxy covering called the
cuticle. This layer's primary function is to prevent water loss within
the leaf. (Plants that leave entirely within water do not have a
cuticle).
• Directly underneath the cuticle is a layer of cells called the
epidermis.
• The vascular tissue, xylem and phloem are found within the veins of
the leaf. Veins are actually extensions that run from to tips of the
roots all the way up to the edges of the leaves. The outer layer of the
vein is made of cells called bundle sheath cells, and they create a
circle around the xylem and the phloem. In most veins, xylem is the
upper layer of cells and the lower layer of cells is phloem. Recall that
xylem transports water and phloem transports sugar (food).
• Within the leaf, there is a layer of cells called the mesophyll. The
word mesophyll is Greek and means "middle" (meso) "leaf" (phyllon).
Mesophyll can then be divided into two layers, the palisade layer and
the spongy layer.
• Palisade cells are more column-like, and lie just under the epidermis,
• the spongy cells are more loosely packed and lie between the palisade
layer and the lower epidermis. The air spaces between the spongy
cells allow for gas exchange.
• Mesophyll cells (both palisade and spongy) are packed with
chloroplasts, and this is where photosynthesis actually occurs.
stomata
• Stomata are microscopic pores found
on the under side of leaves. You will
find the stomata in the epidermal
tissue. The stomata is bounded by two
half moon shaped guard cells that
function to vary the width of the pore.
Stomata help regulate the rate of
transpiration
• About 90% of the water a plant loses escapes through stomata
• open
–
–
Increase photosynthesis
Increase water loss through stomata
• closed
–
–
Decrease water loss through transpiration
Decrease gas exchange and reduce photosynthesis
20 µm
Figure 36.14