Chapter 35: Plant Structure and Growth
The Plant Body
Both genes and environment affect plant structure
Plants have three basic organs: roots, stems and leaves
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The Root System
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Fibrous vs. taproots
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Root hairs
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Adventitious roots: roots arising from the shoot or leaves and
existing above ground.
Figure 35.2 Morphology of a flowering plant: an overview
Figure 35.3 Radish root hairs
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The Shoot System: Stems and Leaves
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Stems
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Nodes, internodes, axillary buds and terminal buds
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Apical Dominance: ability of the terminal bud, through
hormonal action, to inhibit the growth of the axillary buds.
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The concept of “pruning” and “pinching back” plants
Modified shoots
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Stolons: above ground “runners” of strawberry plants;
this is a form of asexual reproduction when
the parent plant fragments.
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Rhizomes: horizontal stems that are underground
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Tubers: starch containing, swollen ends of rhizomes
(onions, potatoes)
Figure 35.4 Modified shoots: Stolons, strawberry (top left); rhizomes, iris (top
right); tubers, potato
(bottom left); bulb, onion (bottom right)
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Leaves
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Blade, petiole
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Grasses don’t have petioles. At the base of the leaf there is
wrapping around of the blade to form a sheath around the
stem.
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Apical Dominance: ability of the terminal bud, through
hormonal action, to inhibit the growth of the axillary buds.
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The concept of “pruning” and “pinching back” plants
Venation
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Monocots have parallel venation in their leaves
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Dicots have a branching of the veins.
Plant organs are composed of three tissue systems: dermal, vascular, and
ground
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Dermal Tissue or epidermis
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Protection of the young parts of the plant
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Can be specialized such as in the formation of root hairs or a
cuticle.
Vascular Tissue
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Xylem
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Tracheids and vessel elements: both dead at maturity
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Wood is mainly tracheids and vessel elements
Figure 35.8 Water-conducting cells of xylem
Figure 35.9 Food-conducting cells of the phloem
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Phloem
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Composed of sieve tube members, sieve plates
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Companion Cells
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nonconducting cell
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connected to a sieve tube by plasmodesmata
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its nucleus and ribosomes service the sieve tube with
which it is associated.
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also help to transport (load) sugar into the sieve tube
members
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Ground Tissue
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tissues that are not vascular or dermal.
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they could be tissues that simply increase the girth of the plant, or
storage cells
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the pith of a plant and its cortex are considered ground tissues.
Figure 35.10 Review of general plant cell structure
Plant tissues are composed of three basic cell types: parenchyma,
collenchyma, and sclerenchyma
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Parenchyma Cells
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Thin, flexible primary cell walls
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Protoplasm (all cell contents except for cell wall) contains a huge
vacuole.
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Least specialized
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Aid in sugar transport, contain chloroplasts, store starch, make up
most of the fruit tissue that you eat.
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Collenchyma Cells
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Thicker primary cell walls than parenchyma cells
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Help support a young plant
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Strings of the celery stalk are made of collenchyma cells
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Living cells, flexible and can elongate
Sclerenchyma Cells
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Thick secondary cell walls
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cell walls contain lignin
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cannot elongate like the collenchyma cells
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dead at maturity
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Support cells are called fibers and sclereids
Figure 35.11 The three major categories of plant cells
The Process of Plant Growth and Development
Meristems generate cells for new organs throughout the lifetime of a plant
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Types of Meristems
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Apical: tips of roots and shoots
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Primary growth: lengthening of the root or shoot
Lateral Meristems: widening of roots and shoots
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a lateral meristem could add bark
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another lateral meristem could add vascular tissue
Figure 35.12 Locations of major meristems: an overview of plant growth
Figure 35.13 Morphology of a winter twig
Primary growth: Apical meristems extend roots and shoots by giving rise
to the primary plant body
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Primary Growth of Roots
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Root Cap
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Three zones of cells at the cap
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Zone of cell division (meristematic region)
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Zone of elongation: pushes root tip through soil
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Zone of maturation: area of root hairs
Figure 35.14 Primary growth of a root
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Roots have 3 types of meristematic tissues
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Protoderm which gives rise to epidermis of the root
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Procambium which produces the stele, which is vascular
tissue.
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Ground Meristem which gives rise to all other types of cells
called ground tissue system. Example: parenchyma cells for
food storage.
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The Epidermis
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single cell layer
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protection
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specialized epidermal cells produce root hair extensions that
absorb water and minerals
The Stele
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central cylinder in roots
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vascular tissue
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has a different organization in dicots and monocots
•Dicots: stele is made of phloem and xylem. The xylem
form spokes with the phloem in between the spokes
•Monocots: stele is parenchyma cells and xylem and phloem
Figure 35.15 Organization of primary tissues in young roots
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The Ground Tissue System
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full of parenchyma cells
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fills the cortex
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where the roots store food
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Endodermis: innermost layer of the ground tissue; surrounds
the stele in monocot and dicot roots
Figure 35.16 The formation of lateral roots
Forms from outermost layer of
the stele, the pericycle. It will
become meristematic and push
through the cortex.
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Primary Growth of Shoots
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3 Primary Meristems (just like in the roots)
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Protoderm: gives rise to the epidermis (just like the root)
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Here the epidermis covers the stems and leaves
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Procambium: gives rise to the vascular tissue
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Ground meristem: gives rise to the ground tissue such as the
pith of a stem and the cortex of the stem.
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Primary Tissues of Stems
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Vascular Bundles run up and down the stems in strands
whereas in roots the vascular bundles are in the vascular
cylinder.
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Xylem faces the inside of the stem (the pith) and the phloem
is to the cortex.
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Monocots:
the vascular bundles of xylem and phloem
are scattered throughout the ground tissue
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Dicots:
the vascular bundles of xylem and phloem
are arranged in rings.
Figure 35.18 Organization of primary tissues in young stems
Figure 35.19 Leaf anatomy
Secondary growth: lateral meristems add girth by producing secondary
vascular tissue and periderm.
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Introduction
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There are two meristematic regions concerned with lateral
growth:
1.
Vascular cambium: produces secondary xylem and phloem
2.
Cork cambium: tough outer covering of stems and roots
and in the process it replaces the epidermis.
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Secondary Growth of Stems
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Vascular Cambium and the Production of Secondary V. Tissue
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accounts for the increase in girth of gymnosperms and dicots
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produces secondary xylem to the interior and secondary
phloem to the exterior.
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It’s the secondary growth of xylem that we call wood.
Figure 35.20 Production of secondary xylem and phloem by the vascular cambium
This occurs NOT at the apical meristem (that’s primary growth where things are
lengthening) but farther down the stem.
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Wood
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Consists mainly of tracheids and vessel elements and
sclereid fibers. All are dead
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These cells have hard, lignified cell walls.
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In temperate regions, where there is a winter and
spring/summer, the vascular cambium stops differentiating
in winter, forming rings.
•Spring tracheids are large because here is lots of water
present which expands the cells before they die.
Figure 35.21 Secondary growth of a stem (Layer 3)
Figure 35.22 Anatomy of a three-year-old stem
Figure 35.22x Secondary growth of a stem
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Cork Cambium and the Production of Periderm
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As the girth increases, the epidermis will split and fall off.
Another meristematic tissue, the cork cambium, produces
cork cells to the exterior.
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Mature cork cells secrete suberin which is the wax coating the
cell walls.
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Cork functions as a pathogen barrier.
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Periderm: cork plus cork cambium
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Lenticels: small openings through the periderm for gas
exchange.
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Bark: all tissues external to the vascular cambium which
includes the secondary phloem, cork cambium and cork but
this phloem does not transport sugar
Figure 35.23 Anatomy of a tree trunk
No water transport here
Consists of
secondary xylem
Consists of
secondary xylem
Mechanisms of Plant Growth and Development
Introduction
•The same questions pertain to plants as they do to animals:
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How does the fertilized egg, the zygote become so many different
types of tissues and organs?
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If they all have the same genome what is regulating these
processes; the branching, the formation of leaves, flowering, the
dropping of leaves.
There are three fundamental processes that will transform the zygote
into a plant:
1.
Growth
2.
Morphogenesis
3.
Differentiation
Mechanisms of Plant Growth and Development
Molecular biology is revolutionizing the study of plants
•The plant most used for research is Arabidopsis thaliana or Arabidopsis
•Arabidopsis has the expected characteristics of an organism you could
work with in the lab:
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small and easily growth in the lab or greenhouse
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short generation time of about 6 weeks so you get new seeds
(offspring) quickly.
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It has a small genome (Arabidopsis has had its genome
sequenced.)
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Knowing its genes and determining their function allows
the scientists to know how plants develop.
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One way to know what a gene does is to mutate the gene
and see what structure or function has been goofed up.
Figure 35.25x Arabidopsis thaliana
Figure 35.25 The proportion of Arabidopsis genes in different functional
categories
Mechanisms of Plant Growth and Development
Growth, morphogenesis, and differentiation produce the plant body
•Growth: simply the increase in size and mass of the plant
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This results from cell division and cell enlargement
•Morphogenesis: the formation of the form of the organism, in this case the
plant, with its leaves, branching pattern, roots, stems and also the
organization of these structures
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So, same question, what genes are being turned on, off, at what
time in development, what genes are being turned on together.
•Differentiation: as far as plants are concerned it would be the formation of
the diversity of cell types, such as guard cells, apical meristems, roots and
root hairs, xylem and phloem.
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How are the genetic instructions regulated to get a healthy plant?
It’s the same as asking that about a human. . How do the genes
get regulated to produce a healthy baby?
Mechanisms of Plant Growth and Development
Growth involves both cell division and cell expansion
•The Plane and Symmetry of Cell Division
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It is important to have cells simply divide in one plane so a plant
gets taller.
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Asymmetrical division is important in the formation of
specialized cells such as guard cells.
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A key factor in controlling this is the distribution of the
cytoplasm into the daughter cells
A Preprophase Band of microtubules can orient itself one of two
ways around the nucleus and control the plane of division.
Figure 35.26 The plane and symmetry of cell division influence development of
form
A totally different plane of division
occurred to produce these two
guard cells.
Figure 35.27 The preprophase band and the plane of cell division
• A Preprophase Band of
microtubules can orient itself
one of two ways around the
nucleus and control the plane
of division.
•Occurs during late interphase
•The microtubules hold the
nucleus in place during
spindle formation so the
spindles are directed to their
respective positions to
produce the determined
division pattern.
Mechanisms of Plant Growth and Development
•The Orientation of Cell Expansion
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The uptake of water is very important and this produces the cell’s
expansion.
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The water goes into the central vacuole which grows due to the
coalescing of many small vacuoles.
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They don’t expand equally in all directions; usually it is along the
main axis which is controlled by the cellulose in the cell walls.
Figure 35.28 The orientation of plant cell expansion
Figure 35.29 A hypothetical mechanism for how microtubules orient cellulose
microfibrils
Mechanisms of Plant Growth and Development
•The Importance of Cortical Microtubules in Plant Growth
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Mutants have been made of Arabidopsis, called Fass mutants.
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The microtubular arrangement in these mutants is abnormal
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The preprophase bands do not form in an organized fashion
which affects the arrangement of the cellulose microfibrils in the
cell wall.
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The poor arrangement of the cellulose microfibrils affects the
elongation of the cell causing it to elongate in all directions
equally and to divide every which way.
The fass mutant of Arabidopsis confirms the importance of cortical
microtubules to plant growth
Wild-Type
Seedling
Fass Seedling
Mature Fass
Mutant
Mechanisms of Plant Growth and Development
Morphogenesis depends on pattern formation
•Pattern Formation: the development of specific structures, like a branch
or a sepal, in a specific location.
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Pattern Formation depends on signals that each cell detects when
it is part of the embryo.
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Even when a cell is within an organ, such as a developing flower,
it continues to receive cues about its role in flower development.
Mechanisms of Plant Growth and Development
•What produces this positional information? What are the cues?
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Polarity
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Remember that a plant has a root and a shoot end and these
two poles of the plant must form very different structures.
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The first cell division of the plant zygote is asymmetrical
and this polarizes the plant into a shoot and a root.
Mechanisms of Plant Growth and Development
Cellular differentiation depends on the control of gene expression
•Cellular differentiation is accompanied by differential protein production
with a cell.
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Cells with the same genome follow different sets of instructions.
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Do you see why the control of transcription and translation is so
important?
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It’s the proteins that are produced by the expression of these
genes that control the plant development.
Cellular differentiation depends on the control of gene expression
A single cortical
cell is in contact
with two
epidermal cells.
This causes a
specific gene to
be expressed and
these cells will not
form root hairs.
These cells are root cap
cells which will be lost as
the root hairs differentiate
from the root cells.
This epidermal
cell touches
two cortical
cells and a
gene is not
expressed and
root hairs will
develop
Mechanisms of Plant Growth and Development
Clonal analysis of the shoot apex emphasizes the importance of a cell’s
location in its developmental fate
Phase changes mark major shifts in development
• Definition: the changing from one developmental phase to another
such as changing of a leaf’s change when it is young to when it is
older. (Figure 35.34)
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Phase changes can occur throughout a plant’s life. An older branch
can produce young or juvenile leaves and make that part of the plant
look young. In animals, this does not happen; the entire organism
ages. You don’t find a young liver in an older person because the
liver developed later in life.
Figure 35.34 Phase change in the shoot system of Eucalyptus
Young leave structure
Older leave structure
Mechanisms of Plant Growth and Development
Genes controlling transcription play key roles in a meristem’s change
from a vegetative to a floral phase.
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What this is referring to is simply, what changes occur when a plant
begins to flower? What goes on cellularly to make the plant produce
a flower?
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Length of night, hormones, and intracellular signals do all this
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The genes associated with these changes are called meristem
identity genes.
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And like all genes, they make proteins, and in this case they
are transcription factors that take the nonflowering part of
the apical meristem and make it flower.
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Some of the genes responsible for forming stamens and
carpels have been identified by mutating certain genes and
seeing what malfunctioned.